1.1: DEFINITION OF CKD
1.1.1: CKD is defined as abnormalities of kidney structure or function, present for
>3 months, with implications for health (Table 2). (Not Graded)
RATIONALE
The definition of CKD remains intact, but we have clarified the classification and
risk stratification as indicated below. The addition of ‘with implications for health'
is intended to reflect the notion that a variety of abnormalities of kidney structure
or function may exist, but not all have implications for health of individuals, and
therefore need to be contextualized.
Kidney damage refers to a broad range of abnormalities observed during clinical assessment,
which may be insensitive and non-specific for the cause of disease but may precede
reduction in kidney function (Table 2). Excretory, endocrine and metabolic functions
decline together in most chronic kidney diseases. GFR is generally accepted as the
best overall index of kidney function. We refer to a GFR <60 ml/min/1.73 m2 as decreased
GFR (Table 2) and a GFR <15 ml/min/1.73 m2 as kidney failure. AKI may occur in patients
with CKD and hasten the progression to kidney failure.
14
Complications include drug toxicity, metabolic and endocrine complications, increased
risk for CVD, and a variety of other recently recognized complications, including
infections, frailty, and cognitive impairment.
15, 16, 17, 18
Complications may occur at any stage, often leading to death without progression to
kidney failure. Complications may also arise from adverse effects of interventions
to prevent or treat the disease and associated comorbidity.
Criteria for CKD
Defining terms: The following section aims to define specific terms and concepts so
as to ensure clarity among all users. In addition, the rationale for including these
terms is included.
Table 3 provides a justification for the criteria for CKD. The criteria for definition
of CKD are objective and can be ascertained by means of simple laboratory tests without
identification of the cause of disease, thereby enabling detection of CKD by non-nephrologist
physicians and other health professionals.
Duration >3 Months
Kidney diseases may be acute or chronic. We explicitly but arbitrarily define duration
of >3 months (>90 days) as delineating “chronic” kidney disease. The rationale for
defining chronicity is to differentiate CKD from acute kidney diseases (such as acute
GN), including AKI, which may require different interventions, and have different
etiologies and outcomes.7 We did not define acute kidney disease (AKD) because there
does not appear be an evidence base for a precise definition.
The duration of kidney disease may be documented or inferred based on the clinical
context. For example, a patient with decreased kidney function or kidney damage in
the midst of an acute illness, without prior documentation of kidney disease, may
be inferred to have AKI. Resolution over days to weeks would confirm the diagnosis
of AKI. A patient with similar findings in the absence of an acute illness may be
inferred to have CKD, and if followed over time would be confirmed to have CKD. In
both cases, repeat ascertainment of kidney function and kidney damage is recommended
for accurate diagnosis. The timing of the evaluation depends on clinical judgment,
with earlier evaluation for the patients suspected of having AKI and later evaluation
for the patient suspected of having CKD. For further details on the Evaluation of
CKD, see Chapter 1.4.
Reversibility.
Most kidney diseases do not have symptoms or findings until later in their course
and are detected only when they are chronic. Most causes of CKD are irreversible with
a life-long course, and treatment aimed at slowing progression to kidney failure.
However, chronicity is not synonymous with irreversibility. In some cases, CKD is
entirely reversible, either spontaneously or with treatment, and in other cases, treatment
can cause partial regression of kidney damage and improvement in function (e.g., immunosuppressive
therapies for GN). Even kidney failure may be reversed with transplantation. Because
of the long course of most cases of CKD, patients often have one or more episodes
of AKI, superimposed upon CKD.
Decreased GFR
The kidney has many functions, including excretory, endocrine and metabolic functions.
The GFR is one component of excretory function, but is widely accepted as the best
overall index of kidney function because it is generally reduced after widespread
structural damage and most other kidney functions decline in parallel with GFR in
CKD.
We chose a threshold of GFR <60 ml/min/1.73 m2 (GFR categories G3a-G5) for >3 months
to indicate CKD. A GFR <60 ml/min/1.73 m2 is less than half of the normal value in
young adult men and women of approximately 125 ml/min/1.73 m2. Figure 2 shows a compilation
of GFR measurements in apparently healthy men and women in the US and Europe by age
from more than 40 years ago.
20
The age-associated GFR decline is observed in longitudinal as well as cross sectional
studies, but varies substantially among individuals within the population.
21
More recent data in kidney donors confirm these general trends.
22, 23
Limited data are available for non-whites in the US and Europe or in other countries,
although data suggest that the normal range for measured GFR and the age-associated
decline is similar.
24, 25, 26
A GFR <60 ml/min/1.73 m2 can be detected by routine laboratory testing. Current estimating
equations for GFR (eGFR) based on serum creatinine (SCr), but not SCr alone, are sensitive
for detecting measured GFR<60 ml/min/1.73 m2.
27
A decreased eGFR using SCr can be confirmed by GFR estimation using an alternative
filtration marker (cystatin C) or GFR measurement, as necessary.
A GFR<60 ml/min/1.73 m2 is associated with a higher risk of complications of CKD than
in subjects with CKD and conserved GFR. The causal mechanisms underlying these associations
are not fully understood. We consider three main types of complications, which are
of relevance to all patients with CKD and reduced GFR, irrespective of country, age
or etiology:
Drug toxicity.
Altered pharmacokinetics of drugs excreted by the kidney and an increased risk of
drug-interactions are common and require adjustment in the dosage of many drugs (see
Chapter 4.4).13 At lower GFR, altered pharmacokinetics and pharmacodynamics of drugs
not excreted by the kidney may also be observed. Errors in drug dosing are common
in patients with CKD and may be associated with toxicity to the kidney (resulting
in AKI) or systemic toxicity, resulting in threats to patient safety.
Metabolic and endocrine complications.
As GFR declines a variety of complications reflecting loss of endocrine or exocrine
function of the kidneys develop including anemia, acidosis, malnutrition, bone and
mineral disorders (described in Chapters 3 and 4).
Risk of CVD and death.
A meta-analysis by the CKD Prognosis Consortium demonstrated associations of eGFR
<60 ml/min/1.73 m2 with subsequent risk of all-cause and cardiovascular mortality,
kidney failure, AKI, and CKD progression in the general population and in populations
with increased risk for CVD.3-5
Figure 3 shows the relationship for total and cardiovascular mortality in general
population cohorts. The risk for all outcomes was relatively constant between eGFR
of 75-105 ml/min/1.73 m2, with a suggestion of a U-shaped curve for total mortality.
The increased relative risk (RR) for all outcomes was significant for eGFR of <60 ml/min/1.73 m2.
Kidney Damage
Damage to the kidney can be within the parenchyma, large blood vessels or collecting
systems, and is most often inferred from markers rather than direct examination of
kidney tissue. The markers of kidney damage often provide a clue to the likely site
of damage within the kidney and in association with other clinical findings, the cause
of kidney disease.
Proteinuria.
Proteinuria is a general term for the presence of increased amounts of protein in
the urine. Proteinuria may reflect abnormal loss of plasma proteins due to a) increased
glomerular permeability to large molecular weight proteins (albuminuria or glomerular
proteinuria), b) incomplete tubular reabsorption of normally filtered low-molecular-weight
proteins (tubular proteinuria), or c) increased plasma concentration of low-molecular-weight
proteins (overproduction proteinuria, such as immunoglobulin light chains). Proteinuria
may also reflect abnormal loss of proteins derived from the kidney (renal tubular
cell constituents due to tubular damage) and lower urinary tract. Albuminuria, tubular
proteinuria and renal tubular cell constituents are pathognomonic of kidney damage.
In addition, findings from experimental and clinical studies have suggested an important
role for proteinuria in the pathogenesis of disease progression of CKD.
28
Albuminuria.
Albuminuria refers to abnormal loss of albumin in the urine. Albumin is one type of
plasma protein found in the urine in normal subjects and in larger quantity in patients
with kidney disease.
For a number of reasons, clinical terminology is changing to focus on albuminuria
rather than proteinuria: a) albumin is the principal component of urinary protein
in most kidney diseases; recent recommendations for measurement of urine proteins
emphasize quantification of albuminuria rather than total protein; b) recent epidemiologic
data from studies around the world demonstrate a strong graded relationship of the
quantity of urine albumin with both kidney and CVD risk; and c) later recommendations
in these guidelines classify kidney disease by level of albuminuria. In this guideline,
we will refer to proteinuria when discussing general concepts and will refer either
to total protein, albumin or other specific proteins when discussing measurements,
patterns, and interpretation of proteinuria.
Albuminuria is a common but not uniform finding in CKD. It is the earliest marker
of glomerular diseases, including diabetic glomerulosclerosis, where it generally
appears before the reduction in GFR. It is a marker of hypertensive nephrosclerosis
but may not appear until after the reduction in GFR. It is often associated with underlying
hypertension, obesity, and vascular disease, where the underlying renal pathology
is not known.
Normative values for albuminuria and proteinuria are generally expressed as the urinary
loss rate. The urinary loss rate of albumin and protein has commonly been referred
to as AER and protein excretion rate (PER), respectively, although in the strict physiological
sense they are not excreted. The terms AER and PER will be retained herein.
We chose a threshold for urinary AER of ≥30 mg/24 hours sustained for >3 months to
indicate CKD. This value is considered to be approximately equivalent to an ACR in
a random untimed urine sample of ≥30 mg/g or ≥3 mg/mmol. The rationale for this threshold
is as follows:
An AER of ≥30 mg/24 hours (ACR≥30 mg/g [≥3 mg/mmol]) is greater than 3 times the normal
value in young adult men and women of approximately 10 mg/24 hours (ACR 10 mg/g or
1 mg/mmol).
An AER of ≥30 mg/24 hours (ACR≥30 mg/g [≥3 mg/mmol]) may sometimes be detectable as
‘trace' using a urine reagent strip, depending on urine concentration, but this is
not a consistent finding until AER exceeds approximately 300 mg/24 hours (ACR≥300 mg/g
[≥30 mg/mmol]). As described later, trace or positive reagent strip values/readings
can be confirmed by ACR, and an elevated ACR can be confirmed by urine AER in a timed
urine collection, as necessary.
An AER≥30 mg/24 hours (ACR≥30 mg/g [≥3 mg/mmol]) is associated with an increased risk
for complications of CKD. A meta-analysis by the CKD Prognosis Consortium demonstrated
associations of an ACR≥30 mg/g (≥3 mg/mmol) or reagent strip 1+ protein with subsequent
risk of all-cause and cardiovascular mortality, kidney failure, AKI, and CKD progression
in the general population and in populations with increased risk for CVD3-5 (Figure
4).
Urine sediment abnormalities.
Formed elements, such as cells, casts, crystals, and microorganisms may appear in
the urine sediment in a variety of disorders of the kidney and urinary tract, but
renal tubular cells, red blood cell (RBC) casts, white blood cell (WBC) casts, coarse
granular casts, wide casts, and large numbers of dysmorphic RBCs are pathognomonic
of kidney damage.
Electrolyte and other abnormalities due to tubular disorders.
Abnormalities of electrolytes and other solutes may result from disorders of renal
tubular reabsorption and secretion. These syndromes are uncommon but pathognomonic
of kidney disease. Often the diseases are genetic without underlying pathologic abnormalities.
Other diseases are acquired, due to drugs or toxins, and are usually with prominent
tubular pathologic lesions.
Pathologic abnormalities directly observed in kidney tissue obtained by biopsy.
Evidence of abnormalities of renal parenchyma in kidney biopsies irrespective of eGFR
or other markers of kidney damage must be acknowledged as an important parameter in
defining kidney damage. The pathologic classification of diseases of the renal parenchyma
reflects the localization of the disease to glomeruli, vessels, tubules and interstitium,
or cysts. Renal biopsies are performed in the minority of CKD patients.
Imaging abnormalities.
Imaging techniques allow the diagnosis of diseases of the renal structure, vessels
and/or collecting systems. Thus, patients with significant structural abnormalities
are considered to have CKD if the abnormality persists for greater than 3 months (note
that this does not include simple cysts and clinical context is required for action).
History of kidney transplantation.
Kidney transplant recipients are defined as having CKD, irrespective of the level
of GFR or presence of markers of kidney damage. The rationale for this designation
is that biopsies in kidney transplant recipients reveal pathologic abnormalities even
in patients without decreased GFR or albuminuria. Kidney transplant recipients have
an increased risk of mortality and kidney outcomes compared to the general population
and they require specialized medical management.
29
Implications for Health
CKD is associated with a wide range of complications leading to adverse health outcomes.
For some complications, the causal pathway between kidney disease and adverse outcomes
is well-known. For these complications, there are clinical practice guidelines for
testing and treatment for modifiable factors to prevent adverse outcomes. Since 2002
a large number of epidemiologic studies have linked decreased GFR and albuminuria
to the risk of adverse health outcomes not previously identified as CKD complications.
The exploration of the mechanisms for the relationships of CKD with these complications
is a rapidly growing topic for basic and clinical research. Because of the high prevalence,
adverse outcomes, and high cost of CKD, especially kidney failure, some countries
have developed public health programs for early identification and treatment of CKD
and its complications. The effectiveness of these programs is being evaluated.
Implications for Clinical Practice and Public Policy
CKD was first defined in the 2002 KDOQI Guidelines and endorsed at subsequent KDIGO
Controversies Conferences with minor modifications.
30, 31
The definition of CKD proposed here is intended for use in clinical practice, research
and public health, and has not changed. Thus, the updated version does not change
any of the initiatives that have been commenced with respect to public policy. We
recognize the variation around the world regarding measurement of urine albumin versus
total protein in clinical practice, and we anticipate variation in implementation
of the guideline until more widespread dissemination of the guideline has occurred.
For additional discussion about methods for ascertainment of urine albumin versus
total protein, see Recommendation 1.4.4 (Evaluation of albuminuria). The implications
of highlighting the importance of albuminuria for general practitioners in evaluation
and prognostication may help with identification and care planning. Nonetheless, a
number of concerns about the definition remain, which are clarified below.
30, 32, 33, 34, 35, 36
Areas of Controversy, Confusion, or Non-consensus and Clarification of Issues and
Key Points
General concerns:
a) The use of single thresholds without consideration of patient specific factors
The use of single thresholds to define decreased GFR and increased AER, without consideration
for cause of disease, age, sex, race-ethnicity and clinical context is consistent
with the use of single thresholds for disease markers to define other chronic non-communicable
diseases, such as hypertension, diabetes, and hypercholesterolemia, that primarily
affect the elderly and are associated with an increased risk for cardiovascular mortality.
Biologic variability and error in ascertainment of GFR and AER can lead to misclassification
and false negative and false positive diagnosis. Furthermore, these single thresholds
appear to differentiate groups of individuals and outcomes, irrespective of specific
patient characteristics in a multitude of studies. However, they correspond to thresholds
for RRs for complications, rather than predictions of absolute risk. Furthermore,
as with any diagnostic tests, findings must be interpreted with considerations of
likelihood of disease based on the clinical context but this should not negate the
application of a standard definition for CKD.
Specific concerns:
b) Relationship of CKD criteria to aging
Epidemiologic studies show an increased prevalence of decreased eGFR and increased
ACR in older subjects. There has been vigorous debate as to whether decreased GFR
or increased ACR in older people represent a disease or “normal aging.” Numerous studies
show pathologic abnormalities associated with aging, including glomerular sclerosis,
tubular atrophy and vascular sclerosis. The cause for this association is not clear
but has been hypothesized to reflect disparate processes, such as vascular disease
or senescence.
37, 38, 39
Irrespective of cause, there appears to be increased risk associated with decreased
eGFR or increased ACR in older people, and for this reason, we consider all individuals
with persistently decreased GFR or increased albuminuria to have CKD. Comparison of
the magnitude of risk to younger individuals is complicated. As with other CVD risk
factors, absolute risk appears to be higher in older than in younger individuals,
but RR appears to be lower.3-5 Note is also made that healthy older individuals do
not necessarily have decreased GFR, so that while one may expect some decline, levels
below 60 ml/min/1.73 m2 in individuals without comorbidity is the exception.
20
c) Isolated decreased GFR without markers of kidney damage
A variety of clinical circumstances are associated with GFR <60 ml/min/1.73 m2 for
>3 months in the absence of known structural alterations. Below are examples of these
conditions and the rationale for considering them as CKD:
Heart failure, cirrhosis of the liver, and hypothyroidism
. Decreased GFR complicates the management of the primary disease and patients with
these disorders with decreased GFR have a worse prognosis than those without decreased
GFR. In addition, renal biopsy in these patients may reveal renal parenchymal lesions.
Kidney donors
. The usual level of GFR in kidney donors after transplantation is approximately 70%
of the pre-donation level, in the range of 60-90 ml/min/1.73 m2 in most donors. However,
a minority of donors have GFR <60 ml/min/1.73 m2. The prognosis of these donors compared
to those with higher GFR has not been carefully studied. However, as with decreased
GFR due to recognized kidney diseases, donors with decreased GFR require closer follow-up
for adjustment of drug doses.
Malnutrition
. The level of GFR is affected by habitual protein intake.40 Healthy adults with lower
protein intake may have lower mean GFR, but usually do not have GFR <60 ml/min/1.73 m2.
Older studies of patients with protein-calorie malnutrition and more recent studies
of subjects with anorexia nervosa have documented reduced measured GFR that can improve
following restoration of nutritional status. However, renal biopsies may reveal structural
abnormalities in these conditions and decreased GFR can complicate their management.
d) Isolated albuminuria without decreased GFR
As described later, transient ACR ≥30 mg/g (≥3 mg/mmol) can occur in disorders other
than CKD. Remission of albuminuria within 3 months in association with recovery from
these disorders is not defined as CKD. Patients with persistent albuminuria would
be considered to have CKD. Below are examples of these conditions and the rationale
for considering them as CKD:
Obesity and metabolic syndrome.
Albuminuria can be associated with obesity and metabolic syndrome, and can remit during
weight loss. The mechanism of albuminuria in these conditions is not known but renal
biopsies may reveal prominent vascular lesions. Patients with obesity and metabolic
syndrome are at increased risk for development of diabetes and hypertension. The risk
of persistent albuminuria in this condition has not been carefully studied.
Orthostatic (postural) proteinuria.
41
Albuminuria may rarely be observed in the upright but not recumbent posture in patients
with the syndrome of postural proteinuria. This condition is not associated with an
increased risk of long-term adverse outcomes but a thorough evaluation is required
to exclude other causes of CKD. Exclusion is generally possible by studying a first
pass early morning urine (EMU) after overnight recumbency: total protein loss of >1000 mg/24
hours is unlikely to be explained by orthostatic proteinuria.
e) Remission of decreased GFR or markers of kidney damage
If decreased GFR and markers of kidney damage resolve while on treatment, the patient
would be considered to have treated CKD, consistent with nomenclature for treated
hypertension, treated diabetes, or treated hypercholesterolemia if blood pressure,
blood glucose and blood cholesterol are within normal range while on medications.
If resolution of decreased GFR and markers of kidney damage is sustained after withdrawal
of treatment, the patient would be considered to have a history of CKD.
f) Kidney disease in the absence of decreased GFR and markers of kidney damage
A GFR ≥60 ml/min/1.73 m2 may reflect a decline from a higher value, and an AER of
<30 mg/24 hours (ACR <30 mg/g or <3 mg/mmol) may reflect a rise from a lower value.
Both findings may be associated with a pathologic process, even in the absence of
other markers of kidney damage. Although such patients do not fulfill the criteria
for CKD, a clinician's high index of suspicion may warrant additional diagnostic testing
or close follow-up to detect the onset of CKD.
Pediatric Considerations
In general the definition of CKD in adults applies to children (birth-18 years) with
the following exceptions or allowances:
the criteria for duration >3 months does not apply to newborns or infants ≤3 months
of age.
the criteria of a GFR <60 ml/min/1.73 m2 does not apply to children <2 years of age
in whom an age appropriate value should be applied.
a urinary total protein or albumin excretion rate above the normal value for age may
be substituted for albuminuria ≥30 mg/24 hours.
all electrolyte abnormalities are to be defined in light of age normative values.
Developmental renal abnormalities account for as many as 30-50% of the children with
CKD or ESRD.
42
As such many infants while born with normal SCr for age will in fact meet the definition
of CKD based on structural abnormalities despite the appearance of a normal GFR and
may be classified as such within the first few days of life.
Normal GFR in newborns is less than 60 ml/min/1.73 m2, and it is not until approximately
2 years of age that one expects to see body surface area (BSA) adjusted GFR values
comparable to those seen in the adult.
43, 44
The expected increases in GFR that occur in the first months of life are due to increases
in mean arterial pressure (MAP), decrease in renal vascular resistance, and redistribution
of intrarenal blood flows to the superficial cortical nephrons in the newborn and
increases in glomerular size and capillary permeability in the infant.
45, 46, 47, 48
As such direct application of the GFR threshold values in the current CKD definition
would not be appropriate in children less than 2 years of age as their normative maximal
values would be below those of the adult or older child; hence most neonates and infants
would be classified a priori at a decreased GFR based not on a reduction in GFR from
a higher value, but rather failure of maturity of the kidney.
Numerous references exist for fetal,
49
neonatal term,
44, 48
pre-term,
46, 50, 51
infant, child and adolescent GFR values
43, 44
and the reader is strongly encouraged to use such references when comparison to a
normative range is required for approximating the reduction in renal clearance of
the individual child. It should be noted that across these ages the method of GFR
measurement has often varied with the majority of such measurements in the neonate
(term or preterm) or infant being derived from urinary collections and creatinine
clearance (CrCl) measurements, whereas the older children and adolescents are often
investigated with exogenous markers including inulin, radionuclides, and other markers
such as iohexol or iothalamate.
The most comprehensive list of GFR based on the gold standard of inulin clearance
and stratified by age for both term and preterm babies and children up to the age
of young adults can be found in Schwartz and Furth's review on GFR measurements and
estimation in pediatric CKD.
52
Similarly, age relevant normative values should be utilized when interpreting urinary
protein (albumin) excretions as well as other important urinary and serum laboratory
values. Such values may be found in a number of pediatric nephrology texts. For neonates
and infants this includes Waters
53
and for post-neonate to young adults, more comprehensive values can be found in Langlois.
54
1.2 STAGING OF CKD
1.2.1: We recommend that CKD is classified based on cause, GFR category, and albuminuria
category (CGA). (1B)
RATIONALE
This statement is worded in this way because a classification encompassing cause and
severity, as expressed by the level of GFR and the level of albuminuria, links to
risks of adverse outcomes including mortality and kidney outcomes. These factors will
therefore guide management of CKD and this recommended classification is consistent
with other classification systems of disease which are based on the general domains
of cause, duration and severity which provide a guide to prognosis. We included only
kidney measures as factors in the classification of kidney disease, although we acknowledge
that factors other than kidney measures, such as level of BP, also affect prognosis
in CKD.
This recommended staging with inclusion of two additional domains represents a revision
of the previous CKD guidelines, which included staging only by level of GFR. Cause
of disease is included because of its fundamental importance in predicting the outcome
of CKD and choice of cause-specific treatments. With inclusion of cause of kidney
disease in the classification, we considered that it was no longer necessary to retain
the use of the letter “T” to refer to kidney transplant recipients. Albuminuria is
included as an additional expression of severity of disease not only because it is
a marker of the severity of injury but also because albuminuria itself strongly associates
with progression of kidney disease. Numerous studies have identified the adverse prognostic
implication of albuminuria irrespective of level of kidney function.
We propose that this classification of CKD by Cause, GFR and Albuminuria, respectively
be referred to as CGA staging. It can be used to inform the need for specialist referral,
general medical management, and indications for investigation and therapeutic interventions.
It will also be a tool for the study of the epidemiology, natural history, and prognosis
of CKD.
Pediatric Considerations
The principles inherent in this guideline are fully applicable to children.
While large scale trials in children relating cause, GFR and albuminuria or proteinuria
are rare, the principles of a multimodal classification in these three spheres should
apply to children.
To date the only large scale trial utilizing a validated exogenously measured GFR
(iohexol) and urinary protein excretion in a well-described cohort of children with
renal disease is the Chronic Kidney Disease in Children (CKiD) trial.
55
They have enrolled over 600 children aged 1-16 years and have described GFR and urinary
proteinuria related outcomes in the areas of neurodevelopment, cognition, behavior,
cardiovascular health and risk, and somatic growth. They have also collected samples
for ongoing and future genetic study. While these data are sparse in relation to overall
adult numbers, this represents one of the largest pediatric nephrology trials. The
use of true measured GFR, the quality and completeness of the data, and the long term
longitudinal follow-up will form the basis for the best evidence-based outcomes in
children with CKD for the foreseeable future. A recent review article by Copelovitch
et al.
56
summarizes the major findings of the trial up to the present time.
1.2.2: Assign cause of CKD based on presence or absence of systemic disease and the
location within the kidney of observed or presumed pathologic-anatomic findings. (Not
Graded)
RATIONALE
This statement has been included so as to ensure that clinicians are alerted to the
fact that CKD is not a diagnosis in and of itself, and that the assignment of cause
is important for prognostication and treatment.
The cause of CKD has been traditionally assigned based on presence or absence of underlying
systemic diseases and location of known or presumed pathologic-anatomic abnormalities.
The distinction between systemic diseases affecting the kidney and primary kidney
diseases is based on the origin and locus of the disease process. In primary kidney
disease the process arises and is confined to the kidney whereas in systemic diseases
the kidney is only one victim of a specific process, for example diabetes mellitus.
Certain genetic diseases cross this boundary by affecting different tissues, e.g.,
adult polycystic kidney disease. The location of pathologic-anatomic findings is based
on the magnitude of proteinuria, findings from the urine sediment examination, imaging,
and renal pathology. Table 4 represents an example of a classification of causes of
kidney diseases based on these two domains.
There is wide geographic variation in the cause of kidney disease. In developed countries,
hypertension and diabetes are the most frequent causes of CKD, especially in the elderly.
In populations with a high prevalence of diabetes and hypertension, it can be difficult
to distinguish CKD due to hypertension and diabetes from CKD due to other disorders.
In other countries, other causes of CKD may be as frequent as hypertension and diabetes
(e.g., glomerular disease in East Asia) or coexist with them. Specialized diagnostic
testing, such as kidney biopsy or invasive imaging studies are performed only when
it is essential to confirm some diagnoses and the benefits justify the risks and cost.
It is anticipated that cause of disease will not be known with certainty for many
patients with CKD but can be either inferred or not known.
Pediatric Considerations
The principles inherent in this guideline are fully applicable to children.
1.2.3: Assign GFR categories as follows [Table 5] (Not Graded):
RATIONALE
The purpose of this statement is to ensure clarity in communication. The terms associated
with each of the GFR categories are descriptors which need to be taken in the context
of the individual and are all references to normal young adults. Note that mildly
decreased kidney function (G2) in the absence of other markers, does not constitute
CKD.
The associations of lower categories of GFR and risks of metabolic and endocrine complications
formed the basis of the previous stratification into 5 stages. This current classification
further acknowledges the importance of dividing Stage 3 based on data supporting different
outcomes and risk profiles into categories G3a and G3b (Figure 5). A number of other
concurrent complications are associated with decreased categories of GFR including
infection, impaired cognitive and physical function, and threats to patient safety.
57
Figures 6 and 7 detail the RRs of decreased eGFR and increasing ACR with future complications,
including mortality and kidney outcomes.
30
Even for the group with the lowest value of albuminuria, the increased RR for all
outcomes is significant for eGFRs below 60 ml/min/1.73 m2 in the continuous analysis
and in the range of 45–59 ml/min/1.73 m2 for the categorical analysis.
Pediatric Considerations
In children <2 years of age with CKD, the GFR categories as per the adult in Table
5 do not apply; these children should be categorized as having normal, moderately
reduced, or severely reduced age-adjusted GFR.
No currently agreed upon set of international normative values or categories exist
for GFR in children under the age of 1-2 years. However, the international pediatric
nephrology community has embraced the adult CKD staging system as per the 2002 KDOQI
guidelines in children over the age of 2 years, as suggested by Hogg et al.
43
As indicated in Pediatric Considerations for Guideline 1.1, the normative GFR values
for children less than 2 years vary quite widely by both age and method of measurement.
More importantly these values are expected to increase in a non-linear fashion over
the first 2 years of life with significant changes seen in the first few months post-birth
and no current evidence of presence of comorbid conditions at any given level of measured
or estimated GFR in this population. As such, specific categorization of G1-5 as suggested
in this Recommendation would seem not be of value, and might be misleading if applied
to a child less than 2 years of age.
With this in mind, it is suggested that based on the chosen method of GFR measurement
or comparison for the individual (i.e., CrCl, radioactive or cold exogenous serum
markers, or estimating formula), that one should attempt to classify the child under
the age of 2 years as having normal, moderate or severe reductions in GFR based on
the normative range and standard deviations (SDs) for the method. No evidence exists
for this recommendation but recognition that values of GFR more than 1 SD below the
mean would seem likely to raise concern of the clinician and foster the need for closer
monitoring. For drug dosing adjustments it is suggested that those children with GFRs
below the mean by >1 but <2 SD be classified as having a moderate reduction in GFR
whereas those more than 2 SD below the mean for the method be classified as having
a severe reduction in GFR.
1.2.4: Assign albuminuria* categories as follows [Table 6] (Not Graded):
*note that where albuminuria measurement is not available, urine reagent strip results
can be substituted (Table 7)
RATIONALE
The purpose of this statement is to ensure communication and to reflect that albuminuria
category is an important predictor of outcomes. The association of high levels of
proteinuria with signs and symptoms of nephrotic syndrome is well known. The detection
and evaluation of lesser quantities of proteinuria have gained additional significance
as multiple studies have demonstrated its diagnostic, pathogenic, and prognostic importance.
There is a continuous risk associated with albuminuria but the use of a simple categorical
approach was selected to simplify the concept for clinical practice. Several groups
had suggested subdividing one or more GFR categories based on albuminuria category.
For the detection of diabetic nephropathy some guidelines recommend the use of different
ACR thresholds for males and females (>25 mg/g [>2.5 mg/mmol] and >35 mg/g [>3.5 mg/mmol],
respectively) to take into account variations in creatinine excretion. A single threshold
is used in North America (30 mg/g or 3.4 mg/mmol). Earlier KDIGO guidance was reluctant
to adopt gender-specific thresholds due to greater complexity, uncertainty about assay
precision, and effects of race, ethnicity, diet and measures of body size on creatinine
and this stance is maintained here. For simplicity, and to reflect the fact that it
is an approximation, 3.4 mg/mmol as the current guideline threshold has been rounded
to 3.0 mg/mmol.
There is a graded increase in risk for higher albuminuria categories, at all GFR categories,
without any clear threshold value. Even for subjects with GFR >60 ml/min/1.73 m2,
the increased RR is statistically significant for urine ACR ≥30 mg/g (≥3 mg/mmol)
for mortality and kidney outcomes (Figures 6 and 7). The predictive ability of albuminuria
at all categories of GFR supports the suggestion to add albuminuria categories to
all GFR categories. Since the relationship with albuminuria is continuous, the selection
of the number of categories and the cutoff values appears arbitrary. The Work Group
has recommended the classification of albuminuria into only 3 categories, based on
practical considerations, but recognized that further subdivisions within the category
of <30 mg/24 hours (ACR<30 mg/g or <3 mg/mmol) may be useful for risk stratification,
and that subdivisions within the category of >300 mg/24 hours (ACR>300 mg/g or >30 mg/mmol)
may be useful for diagnosis and management. Specifically there is a recognition that
nephrotic range proteinuria (AER>2200 mg/24 hours [ACR>2200 mg/g;>220 mg/mmol] PER>3000 mg/24
hours [>3000 mg/g;>300 mg/mmol]) confers unique additional risks and is usually associated
with specific conditions (such as GN). As these are relatively rare in general practices,
the simplicity of the AER categorization was preferred. Table 7 shows the approximate
relationships of categories of AER to other measures of albuminuria and proteinuria.
Implications for Clinical Practice and Public Policy
Data from around the world suggest that CKD prevalence is between 10-16% but information
concerning population prevalence by category of GFR and ACR is scant. Figure 8 shows
the proportion of adults in the US by categories of GFR and albuminuria.
19
While CKD is common, few individuals have severely reduced GFR or kidney failure or
severely increased albuminuria.
The classification of kidney disease by cause, category of GFR and category of albuminuria
does not conform to the International Classification of Diseases (ICD) maintained
by the World Health Organization (WHO). Currently the WHO is developing an update
(ICD 11). It will be important to communicate and coordinate efforts with the kidney
disease subgroup for ICD 11. However, the proposed current classification does address
the need in clinical practice to acknowledge the multiple dimensions and variables
by which individual patients are assessed. Table 8 gives examples of the use of CGA
nomenclature.
Definition of GFR categories have been deliberately based upon the concept of “true”
GFR, whereas clinical practice and research has predominantly used creatinine-based
estimates of GFR. The belief of the Work Group is that the non-GFR determinants of
creatinine and the imprecision of creatinine-based GFR estimates have resulted in
the absence of strong dose-dependent association of eGFR with clinical outcomes in
the GFR range of >60 ml/min/1.73 m2. The Work Group felt confident that GFR levels
of ≥90 ml/min/1.73 m2 portend better prognosis than GFR levels 60-89 ml/min/1.73 m2,
if they could be estimated accurately. Therefore, the GFR categories include separate
G1 (≥90 ml/min/1.73 m2) and G2 (60-89 ml/min/1.73 m2) designations despite limited
data from creatinine-based estimates that prognosis differs between these two categories.
It is also an acknowledgement that the degree of precision of some of our measurements
may not be able to differentiate between these 2 categories reliably. As described
later, studies that have used cystatin C have found gradients in prognosis at eGFR
levels above 60 ml/min/1.73 m2, which supports the belief of the committee that separating
these 2 GFR categories is appropriate for CKD classification.
Albuminuria categories are “wide” with respect to risk, with significant gradients
within each category. The decision to propose only 3 categories is based on the perceived
need for simplification in clinical practice. In specialized clinical nephrology centers,
A3 (>300 mg/g or >30 mg/mmol) is often more precisely assessed and divided into additional
categories. For example, nephrotic range proteinuria is defined as PER>3500 mg/24
hours or PCR (protein-to-creatinine ratio) >3500 mg/g [>350 mg/mmol] which is approximately
equivalent to AER>2200 mg/24 hours or ACR>2200 mg/g [220 mg/mmol]. It is clearly recognized
that these very high levels of proteinuria carry a different risk than lower values
within the same category. Further differentiation after quantification and evaluation
would inform treatment decisions for an individual patient. These categories serve
as an initial assessment and prognostication tool; further classification is appropriate
for specific circumstances and is not limited by the initial classification into only
3 categories.
Note that the term ‘microalbuminuria' is not used and is discouraged in this classification
system. This will require a formal education program and review of existing guidelines
in other disciplines so that consistency of terminology and understanding of the changes
are universal (see Recommendation 1.4.4.2.1).
Pediatric Considerations
This statement would need to be altered for application in pediatric practice in the
following way. In children with CKD any expression of abnormal urinary protein excretion,
irrespective of the marker:
must account for variation in that measurement as seen across age, sex, puberty and
or body size (height, weight, body mass index [BMI]).
should account for the possibility of tubular versus glomerular proteinuria dominance
dependent on the underlying disease.
may utilize proteinuria in place of albuminuria.
There is no set standard encompassing all children with respect to the normal range
of urinary protein (or albumin) excretion. Values vary across age, sex, race, pubertal
status, the presence of obesity (high BMI) and may be modified by exercise, fever,
and posture.
60, 61, 62, 63
In general, neonates and young infants/ children are both expected and allowed to
have higher urinary losses of both glomerular and tubular proteinuria due to lack
of maturation in the proximal tubular reabsorption of proteins. The rough equivalences
for ACR and PCR quoted in the pediatric literature are similar, but not identical
to those quoted in the adult literature. Normal ranges vary but at least one reference
suggests as much as 6-8 mg/m2/hr or >240 mg/m2/day of proteinuria as being acceptable
at <6 months of age;
64
normal ranges for urinary albumin losses are not known at this age.
The normal range of protein excretion for children 6-24 months of age in a 24-hour
urine collection is quoted as being <4 mg/m2/hr (<150 mg/m2/day), whereas the first
morning spot urine protein sample is said to be normal at levels of <500 mg/g creatinine
(<50 mg/mmol). In children older than 24 months these values are <4 mg/m2/hr (<150 mg/m2/day)
for the 24-hour collection and PCR <200 mg/g creatinine (<20 mg/mmol) in the first
morning urine sample, or a first morning urine ACR <30 mg/g (<3 mg/mmol).
43, 65
At all ages, total urinary protein excretion >40 mg/m2/hr (>3 grams/1.73 m2/day) is
considered to represent ‘nephrotic range' loss of protein, with intermediate values,
i.e., 4-40 mg/m2/hr or its equivalent representing abnormal but ‘non-nephrotic' losses.
43, 65
Children older than 24 months of age are expected to achieve normal (‘adult') urinary
protein values with the caveat of an exaggerated postural loss of glomerular proteins
(albumin) as can commonly be seen in the 2-5% of the adolescent population (i.e.,
orthostatic proteinuria).
62
Based on National Health and Nutrition Examination Survey III (NHANES III) data from
just under 6000 healthy 6-19 year old children using either immunonephelometry or
radioimmunoassay, the definition of urinary albumin excretion was determined to be
30-300 mg/24 h collection; 20-200 μg/min in an overnight collection and 30-300 mg/g
creatinine (3-30 mg/mmol) in a first morning urine sample.
66
Of note, to date the majority of studies that have examined the effects of urinary
protein losses or therapeutic interventions have concentrated on so-called total protein
excretion or random or first morning PCRs. The utility of measuring the albumin only
fraction, and in particular quantitating this at the lower level of detection, i.e.,
<30 mg/g (<3 mg/mmol) creatinine, is only now being investigated in more detail in
large pediatric studies. As such it should be recognized that in children the quantification
of total protein, as compared to the albumin only fraction, may be the preferred method
for assigning risk as it relates to the presence of urinary protein loss.
In summary, for children older than 2 years of age the assignment of ‘proteinuria'
categories can be used as per the adult guidelines with the understanding that modification
to the upper limit of expected values may be necessary in consideration of the factors
outlined above. Although there is a preference for reporting albumin values, currently
many clinicians still categorize these children based on total protein and in the
child <2 years of age or the adolescent with demonstrable orthostatic proteinuria,
the current albuminuria categories are unlikely to apply.
1.3: PREDICTING PROGNOSIS OF CKD
1.3.1: In predicting risk for outcome of CKD, identify the following variables: 1)
cause of CKD; 2) GFR category; 3) albuminuria category; 4) other risk factors and
comorbid conditions. (Not Graded)
1.3.2: In people with CKD, use estimated risk of concurrent complications and future
outcomes to guide decisions for testing and treatment for CKD complications (Figure
9). (Not Graded)
1.3.3: In populations with CKD, group GFR and albuminuria categories with similar
relative risk for CKD outcomes into risk categories (Figure 9). (Not Graded)
RATIONALE
These statements are worded in this way because for all CKD complications, prognosis
will vary depending on: 1) cause; 2) GFR; 3) degree of albuminuria; and 4) other comorbid
conditions. The relative strength of each of these factors will vary for each complication
or outcome of interest. Risk for kidney disease end points, such as kidney failure
and AKI, is predominately driven by an individual patient's clinical diagnosis, GFR,
and the degree of albuminuria or other markers of kidney damage and injury. For CVD,
risk will be determined by history of CVD and traditional and non-traditional CVD
risk factors. For other conditions, the risk will be determined by risk factors specific
for those conditions. For all conditions, the cause of CKD, GFR category, and albuminuria
category will still have important influence as “risk multipliers,” but will have
smaller overall influence on disease prediction than risk factors specific for the
condition. All these conditions have an impact on life expectancy and quality of life
(QOL) and contribute substantially to predicting the prognosis of CKD. CKD is associated
with numerous complications directly or indirectly related to the cause of CKD, decreased
GFR, or albuminuria (Table 9).
The risk associations of GFR and albuminuria categories appear to be largely independent
of one another. Therefore, neither the category of GFR nor the category of albuminuria
alone can fully capture prognosis for a patient with CKD. The magnitude and gradients
of risk across categories of GFR and albuminuria will likely differ for each specific
adverse event. This heterogeneity across the GFR and ACR grids in RRs for different
outcomes makes it impractical to have a simple hierarchical staging of prognosis across
all cells. Thus, the staging using CGA should be descriptive, but encompassing the
ordered categories of GFR and ACR (Figure 9).
The CGA staging system proposed in this guideline provides a framework for future
recommendations on CKD clinical management. At present, much of the evidence on clinical
decision making in CKD is based solely on GFR. This recommendation serves to highlight
the multidimensional aspect of CKD so as to ensure appropriate consideration of the
complexity of the condition.
Evidence Base
The evidence base from which these statements are derived includes large observational
cohort studies from diverse populations. For some outcomes, including mortality, CVD,
and kidney disease progression, meta-analyses have summarized the risk associations.
For outcomes that occur predominately in older adults (e.g., dementia, fracture),
the evidence is largely limited to cohorts of older people.
Extensive work by the CKD Prognosis Consortium has defined the RRs across GFR and
albuminuria categories for several important outcomes, including all-cause mortality,
CVD, and kidney failure (Figures 6 and 7). Risk increases incrementally in both directions
- down the GFR categories and across the albuminuria categories. Levels of risk can
be identified and grouped into categories, but they may differ somewhat for each outcome.
Additional research is needed to map these GFR and albuminuria categories and cause
of kidney disease to other important outcomes of CKD (Table 9).
International Relevance
The above statements appear to be robust when applied in North America, Europe and
Asia.
30
Thus, it appears for all methods used to determine GFR and to detect albuminuria,
the use of the 3 parameters (cause, category of GFR and category of albuminuria) influences
prognosis irrespective of ethnicity or country of origin.
Implications for Clinical Practice and Public Policy
Providers must incorporate cause of kidney disease, GFR category and albuminuria category
in order to better develop an accurate assessment of an individual's prognosis related
to CKD. Many providers who are not nephrologists will need guidance in the local methods
for requesting and interpreting a urine albumin assessment and an eGFR. Use of risk
scores which are being developed and refined is advised.
Public policy and estimates of total burden of illness in a community need to take
into account the incidence and prevalence of specific conditions (such as diabetes
and congestive heart failure). In addition, knowledge of distribution of levels of
eGFR and ACR may be valuable for resource planning. Community or health-system based
interventions to reduce the incidence of kidney failure in populations should be targeted
and prioritized based on these 3 criteria.
The primary impact on clinical practice will relate to kidney-specific complications
of CKD and referral patterns to help prevent and manage them. Decisions related to
screening and monitoring CKD disorders will be informed and guided by the CGA system.
At present, this evidence for issues such as management of anemia, CKD bone and mineral
disorders, and acid-base disorders has not been organized and presented in this way.
Decisions on screening and referral strategies have major impact on the costs and
quality of health-care. The value of this revised system of classification is that
it will allow the evaluation of different referral patterns and the impact of treatment
strategies in those with diverse CGA assignment. In this way, we will develop additional
evidence which will inform practice patterns. These will necessarily be developed
locally and reflect the values and economic realities of each health-care system.
Areas of Controversy, Confusion, or Non-consensus and Clarification of Issues and
Key points
Current clinical practice has not overtly incorporated these 3 variables into all
decision making activities. The utility of the system will need to be vetted by those
referring and those to whom patients are referred. The overt description of the 3
dimensions of diagnosis and staging of kidney disease which include the cause, the
category of GFR and the category of albuminuria, should help to inform referral and
treatment patterns of large groups of individuals. Risk calculators for specific events
are under development.
The CGA classification system will be useful for quantifying risk for specific outcomes
of CKD but its utility has not been fully assessed in clinical practice and research
studies.
Additional evidence is required before decisions on screening, monitoring, and referral
patterns can be fully informed.
Pediatric Considerations
For Recommendation 1.3.1 the rationale and principles behind this statement would
apply to pediatrics though the data are not available.
Unlike in adults, the knowledge of risk of progression or outcomes of CKD is less
robust in children, with the majority of such information gleaned from either registry
datasets or longitudinal trials. In a 2008 report of a select group of patients enrolled
by various North American pediatric nephrology centers in the North American Pediatric
Renal Trials and Collaborative Studies (NAPRTCS) registry, 46% of nearly 7100 cases
had reached a final ‘end point' with 86% progressing to ESRD over their time in the
registry.
69
Data from the prospective registry and population-based Italian Pediatric Registry
of Chronic Renal Failure (ItalKid) study demonstrated a risk of progression to ESRD
of ∼68% by age 20 years.
70
Cause of CKD.
Specific information related to rate of progression for all pediatric causes of CKD
is not easily available. However data from the prospective longitudinal CKiD trial
has demonstrated a more rapid decline in renal function in children whose underlying
cause of CKD is classified as glomerular with an annualized rate of change in iohexol
GFR of −10.5% as compared to those with a non-glomerular cause in whom the annualized
rate of change is only −3.9%.
71
In terms of absolute rates of change in measured iohexol GFR this translated, in a
separate analysis from the same dataset, into a median change of GFR of −4.3 ml/min/1.73 m2
versus −1.5 ml/min/1.73 m2 in the glomerular versus non-glomerular groups, respectively.
72
This paper also provides the only current individual disease-specific estimate of
annual decline in a pediatric population. Table 10 illustrates that the median values
for annualized change in GFR for various diagnosis categories.
Similarly, a randomized controlled trial (RCT) from Europe
73
examining the effects of diet on rate of progression demonstrated a statistically
significant difference in CrCl between their glomerular and non-glomerular cohorts
at 2 years of follow-up; with the mean decline [SD] in the glomerular group being
−10.7 [11.3] versus −8.4 [13.5] ml/min/1.73 m2 in the non-glomerular patients (P =
0.048).
GFR category.
It is also well recognized that there is an inverse relationship between the rates
of progression of kidney disease to the level of kidney function present at that presentation
with more rapid decline seen in patients with lower initial levels of GFR. Staples
et al.
74
in their retrospective review of the NAPRTCS CKD database involving nearly 4200 children
registered with GFR categories G2-G4 (GFR 15-89 ml/min/1.73m2) demonstrated significantly
higher rates of progression, defined by progression to GFR category G5 (GFR <15 ml/min/1.73m2)
or initiation of dialysis or transplant, for children in GFR categories G3a-G4 (GFR
15–59 ml/min/1.73m2) as compared to those with CKD and GFR category G2 (GFR 60–89 ml/min/1.73m2)
at time of enrollment: hazard ratio (HR) of GFR categories 3a and 3b (GFR 30–59 ml/min/1.73m2)
(GFR category 2 (GFR 60–89 ml/min/1.73m2) = 1.00 as referent): 2.00; 95% confidence
interval (CI) 1.64-2.42; P<0.0001 and HR of GFR category 4 (GFR 15–29 ml/min/1.73m2):
6.68; 95% CI 5.46-8.18; P<0.0001.
Albuminuria (proteinuria).
Several studies have also demonstrated the effect of proteinuria on rate of progression
of CKD in children. Using registry data, and in non-glomerular conditions the ItalKids
trial
75
demonstrated a significantly slower decline in CrCl in patients with baseline PCRs
of <200 mg/g (20 mg/mmol) and 200–900 mg/g (20–90 mg/mmol) when compared to those
patients with a PCR of >900 mg/g (>90 mg/mmol); slope +0.16±3.64 and −0.54±3.67 versus
−3.61 ±5.47 (P <0.0001). This translated to higher rates of kidney survival over 5
years in the lower proteinuria groups, 96.7% and 94.1% versus 44.9%, (P <0.01). Multivariate
analysis confirmed that the baseline PCR correlated with a more rapid decline in CrCl
for any given level of baseline function.
In a prospective multicenter randomized trial of protein intake on rates of progression
in children aged 2-18 years of age, Wingen et al. employed the Schwartz equation to
estimate CrCl and demonstrated that baseline proteinuria in multivariate analysis
was the most important independent predictor of change in CrCl. The authors reported
a partial R2 of 0.259 at 2 years follow-up and similar results were found after the
study was extended for a third year.
73
Life-table analysis in this study also suggested a cutoff value of 50 mg/kg/day of
proteinuria as a strong predictor of time to a decline in CrCl>10 ml/min/1.73 m2 and
found a risk ratio of 4.01 (95% CI 2.23–7.25; P<0.001).
Finally Wong et al.
76
used cross sectional data from the prospective longitudinal CKiD trial to demonstrate
that even after controlling for age, race, BMI, cause of CKD and use of RAAS antagonists
they could expect an average decline in measured GFR of 10% for every increase in
urinary PCR of 14% (95% CI 10-18%).
Other risk factors and comorbid conditions.
Many other risk factors and comorbid conditions have also been associated with greater
risk of progression of CKD in adults but only a few of these have been convincingly
proven in children due to lack of pediatric prospective trials.
Hypertension is by far the best studied of these risk factors in children, with clear
evidence from multiple sources to document the value of aggressive BP control on slowing
the rate of progression of CKD. Wingen et al.
73
demonstrated the importance of systolic BP in rate of progression in both univariate
and multivariate models. In this study Cox proportional hazards analysis demonstrated
a systolic BP >120 mm Hg was an independent risk for decline in CrCl by >10 ml/min/1.73 m2;
risk ratio was 3.1 (95% CI 1.74-5.53; P<0.001).
The most important prospective pediatric BP trial to date, the Effect of Strict Blood
Pressure Control and ACE-Inhibition on Progression of Chronic Renal Failure in Pediatric
Patients (ESCAPE) study, used ambulatory BP monitoring (ABPM) and a fixed dose of
ramipril plus additional antihypertensive agents that do not target the RAAS to assess
(as primary outcomes) the time to decline of 50% in GFR or development of ESRD. Their
results demonstrated a 35% reduction in the risk of achieving the primary end point
in the more intensely treated BP: HR 0.65; 95% CI 0.44-0.94; P=0.02. Further sub-analysis
as reported in the KDIGO BP Guideline10 demonstrated that kidney survival was 66.1%
at 5 year follow-up in patients with systolic BP<90th percentile for age whereas it
was 41% in the patients who did not achieve this level of reduction (P=0.0002); similar
numbers were seen if diastolic BP was the metric considered.
The issue of puberty and its effect on rate of progression has recently been addressed
by the ItalKids investigators.
77
While the methodology of their analysis is less than ideal as they did not determine
actual Tanner stages in the majority of their cohort and used estimated rather than
measured GFR, they do appear to demonstrate a decrease in kidney survival probability
beginning around 10.9 years in girls and 11.6 years in boys with CKD. Of note, the
rate of decline in kidney survival, using these age points as ‘inflection' or break
points, is dramatically increased in both sexes based on their evidence provided in
graphical form, although more precise analyses are not possible from the data provided.
As in adults, other factors for consideration and value in monitoring in children
with respect to risk of progression include obesity, metabolic acidosis, anemia, calcium-phosphate
metabolism, chronic inflammation, diabetes, hyperuricemia, dyslipidemia, and smoking.
The most comprehensive review of many of these factors in children comes from a retrospective
study of the NAPRTCS CKD database. Staples et al.
74
demonstrated that in a multivariate analysis of nearly 4200 children registered with
CKD and GFR categories G2-G4 (GFR 15–89 ml/min/1.73m2), the following factors were
significantly associated with the risk of CKD progression (defined by progression
to GFR category G5 (GFR<15 ml/min/1.73m2) or initiation of dialysis or transplant):
age; primary disease; GFR category; registration year; hypertension; corrected calcium,
phosphorus, albumin, and hematocrit; and as proxies, the use of medications for anemia
and short stature. The ability of this paper to prove causation or value in treating
any of these conditions in hopes of delaying CKD progression is limited by its retrospective
nature, and the fact that data were accrued from a voluntary registry.
There is optimism that prospective data from current large pediatric trials such as
CKiD
55
and the European Cardiovascular Comorbidity in Children with CKD (4C) trial
78
will lead to a better understanding of how risk factors may be influencing the rate
of progression of CKD in children.
For Recommendation 1.3.2 the rationale and principles behind this statement would
apply to pediatrics, though the data are not available. Insufficient evidence currently
exists with respect to the predictive value of prevalent risk factors to guide future
decisions for testing or treatment for CKD complications in an individual child.
It is hoped that well powered, prospective trials with adequate follow-up, such as
the CKiD
55
and European 4C
78
trials, will gather sufficient numbers of patients, comorbidities, and outcomes to
allow for predictive models to be built in pediatric CKD that incorporate traditional
and non-traditional cardiac risk factors including dyslipidemia and hypertension,
proteinuria (albuminuria), specific disease-related issues (e.g., diabetes, tubulopathy),
prematurity, and birth weight.
For Recommendation 1.3.3 the rationale and principles behind this statement would
apply to pediatrics, though the data are not available. Current evidence and a paucity
of numbers do not allow for the statistically relevant categorization of RR for CKD
outcomes based solely on GFR and albuminuria or proteinuria. Again both the CKiD
55
and European 4C
78
trials may be able to address these shortcomings.
1.4: EVALUATION OF CKD
1.4.1: Evaluation of chronicity
1.4.1.1: In people with GFR <60 ml/min/1.73 m2 (GFR categories G3a-G5) or markers
of kidney damage, review past history and previous measurements to determine duration
of kidney disease. (Not Graded)
If duration is >3 months, CKD is confirmed. Follow recommendations for CKD.
If duration is not >3 months or unclear, CKD is not confirmed. Patients may have CKD
or acute kidney diseases (including AKI) or both and tests should be repeated accordingly.
RATIONALE
When evidence of CKD is first ascertained, proof of chronicity can be obtained or
confirmed by:
review of past measurements of GFR;
review of past measurements of albuminuria or proteinuria and urine examinations;
imaging findings such as reduced kidney size and reduction in cortical thickness;
pathological findings such as fibrosis and atrophy;
medical history especially duration of disorders known to cause CKD;
repeat measurements within and beyond the 3 month point.
Chronicity should not be assumed as AKI can present with similar abnormalities.
Pediatric Considerations
See Pediatric Considerations for next section.
1.4.2: Evaluation of cause
1.4.2.1: Evaluate the clinical context, including personal and family history, social
and environmental factors, medications, physical examination, laboratory measures,
imaging, and pathologic diagnosis to determine the causes of kidney disease. (Not
Graded)
RATIONALE
Once the presence of CKD is proven it is essential to establish a cause for this which
will inform specific management and modify risk projections. The diagnosis will be
reached by standard clinical method (i.e., history examination) and special investigation,
based on knowledge of the common causes of CKD and their manifestations. Not all evaluations
are required in all patients, and will be directed by clinical context, and resource
availability. For most patients the following evaluations are indicated:
Reagent strip urinalysis to detect hematuria or pyuria. If positive, use urine microscopy
to detect RBC casts or WBC casts.
Ultrasound to assess kidney structure (i.e., kidney shape, size, symmetry and evidence
of obstruction) as clinically indicated.
Serum and urine electrolytes to assess renal tubular disorders, as clinically indicated.
Many individuals found to have CKD will not have a primary kidney disease but kidney
damage caused by diabetes mellitus, vascular disease, and hypertension. The issue
for the clinician will be to decide whether the presence of these is a sufficient
explanation and if not, to investigate further. The prevalence of other conditions
will vary depending on region, age, and other factors.
It is beyond the scope of this guideline to describe how specific diagnoses are reached
but non-nephrologists in the first instance should review the family history, medications,
symptoms and signs for manifestations of systemic diseases. Urinalysis should be performed,
along with imaging of the kidneys if obstruction of the urinary tract or polycystic
kidney disease is considered.
Pediatric Considerations
For Recommendations 1.4.1.1 and 1.4.2.1, the statements would need to be altered for
application in pediatric practice in the following way.
In any child with GFR<60 ml/min/1.73 m2 (or more than 1 SD below expected for their
age and sex) or with markers of kidney damage, a complete review of their past history
and previous measurement or estimate of renal function and full consideration of the
clinical context, including prenatal history, drug exposures of fetus or mother, genetic
conditions, coincident organ abnormalities, physical examination, fetal and post-natal
laboratory measures including amniotic fluid, pre- and post-natal imaging and pathologic
diagnosis including those of the fetus and placenta should be used to determine the
cause(s) of kidney disease.
As noted in Pediatric Considerations for Recommendation 1.1.1, developmental renal
abnormalities account for as many as 30-50% of the children with CKD.
42
A careful review of all fetal or maternal exposures, genetic risks factors, and any
relevant information on the intrauterine environment during gestation are all relevant
to the determination of the presence of CKD either prior to or present immediately
at the time of delivery. An infant may be born with CKD, leading to immediate classification
within the CGA framework – up to and including that of dialysis dependency.
1.4.3 Evaluation of GFR
This section describes the various methods by which GFR can be estimated. We describe
laboratory techniques that satisfy the requirements for robust result reporting and
we compare the accuracy of available equations for the purpose of reporting eGFR using
a single equation where applicable. We emphasize equations based on standardized measurements
of SCr, but also consider newly developed equations based on standardized measurements
of serum cystatin C (SCysC) because they are being introduced into clinical practice.
We encourage practitioners to have a clear understanding of the value and limitations
of both filtration markers, the importance of standardization of assays for both,
and to understand that when an accurate assessment of kidney function is required,
direct measurement should be undertaken.
1.4.3.1: We recommend using serum creatinine and a GFR estimating equation for initial
assessment. (1A)
1.4.3.2: We suggest using additional tests (such as cystatin C or a clearance measurement)
for confirmatory testing in specific circumstances when eGFR based on serum creatinine
is less accurate. (2B)
RATIONALE
These statements specifically address the need to ensure that estimating equations
are put into routine clinical practice, and that clinicians understand the utility
of further evaluation with additional methods if required.
GFR is measured by the clearance of an exogenous or endogenous filtration marker.
27
All clearance methods are complex so in clinical practice, GFR is estimated from the
serum concentration of the endogenous filtration marker creatinine. Cystatin C is
an alternative endogenous filtration marker; other filtration markers are also under
evaluation. The principles of GFR estimation are discussed in the rationale for recommendations
regarding the use of creatinine as a filtration marker but the concepts apply to GFR
estimation from all endogenous filtration markers. Specific comments about GFR estimation
using cystatin C are presented separately.
For most clinical circumstances, estimating GFR from SCr is appropriate for diagnosis,
staging, and tracking the progression of CKD. However, like all diagnostic tests,
interpretation is influenced by varying test characteristics in selected clinical
circumstances and the prior probability of disease. In particular, an isolated decreased
eGFR in otherwise healthy individuals is more likely to be false positive than in
individuals with risk factors for kidney disease or markers of kidney damage. Confirmation
of decreased eGFR by measurement of an alternative endogenous filtration marker (cystatin
C) or a clearance measurement is warranted in specific circumstances when GFR estimates
based on SCr are thought to be inaccurate and when decisions depend on more accurate
knowledge of GFR, such as confirming a diagnosis of CKD, determining eligibility for
kidney donation, or adjusting dosage of toxic drugs that are excreted by the kidneys.
79
The choice of confirmatory test depends on the clinical circumstance and the availability
of methods where the patient is treated.
Pediatric Considerations
For Recommendation 1.4.3.1, the statements would need to be altered for application
in pediatric practice in the following way. The use of SCr and a recently derived
pediatric specific GFR estimating equation, which incorporates a height term,
80
is preferred over the use of SCr alone in the initial assessment of pediatric renal
function.
For Recommendation 1.4.3.2, this guideline is fully applicable in pediatrics.
1.4.3.3: We recommend that clinicians (1B):
use a GFR estimating equation to derive GFR from serum creatinine (eGFRcreat) rather
than relying on the serum creatinine concentration alone.
understand clinical settings in which eGFRcreat is less accurate.
RATIONALE
Estimating GFR from the SCr concentration alone requires implicit judgments that are
difficult in routine clinical care, including reciprocal transformation, consideration
of the non-GFR determinants, and conversion to the GFR scale. Using GFR estimating
equations provides a more direct assessment of GFR than SCr alone. The SCr concentration
is influenced by GFR and other physiological processes, collectively termed “non-GFR
determinants,” including creatinine generation by muscle and dietary intake, tubular
creatinine secretion by organic anion transporters, and extrarenal creatinine elimination
by the gastrointestinal tract (Figure 10).
GFR estimating equations are developed using regression to relate the measured GFR
to steady state SCr concentration and a combination of demographic and clinical variables
as surrogates of the non-GFR determinants of SCr. By definition, GFR estimates using
SCr concentration are more accurate in estimating measured GFR than the SCr concentration
alone in the study population in which they were developed. Sources of error in GFR
estimation from SCr concentration include non-steady state conditions, non-GFR determinants
of SCr, measurement error at higher GFR, and interferences with the creatinine assays
(Table 11). GFR estimates are less precise at higher GFR levels than at lower levels.
The clinician should remain aware of caveats for any estimating equation which may
influence the accuracy in a given individual patient.
Because of the physiologic and statistical considerations in developing GFR estimating
equations, GFR estimates are less precise at higher GFR levels than at lower levels.
In principle, equations based on multiple endogenous filtration markers can overcome
some of the imprecision of GFR estimates at higher levels, due to cancellation of
errors from non-correlated non-GFR determinants.
Pediatric Considerations
This guideline is fully applicable in pediatrics.
1.4.3.4: We recommend that clinical laboratories should (1B):
measure serum creatinine using a specific assay with calibration traceable to the
international standard reference materials and minimal bias compared to isotope-dilution
mass spectrometry (IDMS) reference methodology.
report eGFRcreat in addition to the serum creatinine concentration in adults and specify
the equation used whenever reporting eGFRcreat.
report eGFRcreat in adults using the 2009 CKD-EPI creatinine equation. An alternative
creatinine-based GFR estimating equation is acceptable if it has been shown to improve
accuracy of GFR estimates compared to the 2009 CKD-EPI creatinine equation.
When reporting serum creatinine:
We recommend that serum creatinine concentration be reported and rounded to the nearest
whole number when expressed as standard international units (μmol/l) and rounded to
the nearest 100th of a whole number when expressed as conventional units (mg/dl).
When reporting eGFRcreat:
We recommend that eGFRcreat should be reported and rounded to the nearest whole number
and relative to a body surface area of 1.73 m2 in adults using the units ml/min/1.73 m2.
We recommend eGFRcreat levels less than 60 ml/min/1.73 m2 should be reported as “decreased.”
RATIONALE
The statement is worded this way to acknowledge that calibration of assays is essential
to interpretation of kidney function measures. This recommendation is directed to
laboratories with the intent to clarify the details of such calibration and the use
of specific equations so as to facilitate international standardization.
81
There are numerous assay methods for creatinine for use in clinical laboratories.
Variation in assigned values for SCr concentration among methods is greater at low
concentrations, corresponding to high levels of GFR. Variation in assays at low SCr
concentrations contributes to imprecision of GFR estimates at high GFR levels.
Currently available assays fall into two broad categories, the alkaline picrate (Jaffe)
assay and enzymatic assays. In general, enzymatic assays are less biased compared
to a standardized reference material and less susceptible to interferences. All assays
are available on a number of platforms.
We recommend that laboratories use assays that are traceable to pure creatinine standards
via a valid calibration hierarchy and that are specific and minimally-biased compared
with isotope-dilution mass spectrometry (IDMS) reference method results. Results should
be traceable to reference materials and methods listed on the Joint Committee for
Traceability in Laboratory Medicine (JCTLM) database. Ideally laboratories should
move to enzymatic assays for creatinine measurement: as a minimum, the use of traditional
kinetic or end point Jaffe assays should cease and be replaced with IDMS aligned Jaffe
methods.
Clinical laboratory information systems generally have access to patient age and sex
and thus can report eGFR based on SCr age and sex, thus providing the clinician with
the test result in units which are recommended for interpretation. Estimated GFR is
now reported together with SCr when creatinine is ordered in more than 75% of clinical
laboratories in the US.
82
In the UK, 93% of NHS laboratories report eGFR with SCr,
83
as is the case in Australia, Canada, and many European countries.
Selection of a single equation for use, where applicable, would facilitate communication
among providers, patients, researchers and public health officials. Criteria for selection
should be based on accuracy compared to measured GFR and usefulness in clinical care
and public health.
The interpretation of measured and eGFR is based on comparison to normative values,
which are adjusted for BSA because of the physiologic matching of GFR to kidney size,
which is in turn related to BSA. The value of 1.73 m2 reflects the average value of
BSA of 25-year old men and women in the USA in 1927.
84
While it is known that modern populations may have different normal values for BSA,
the 1.73 m2 value will be maintained for normalization purposes.
Drug dosing should be based on GFR which is not adjusted for BSA. The effect of drug
dosing based on GFR adjusted for BSA compared to GFR unadjusted for BSA has not been
studied rigorously and more precise recommendations are not available.
Flagging decreased values for eGFR can alert clinicians to the possibility of AKD
or CKD, and may indicate the need for additional investigations or treatments, including
adjustment of doses of drugs that are excreted by the kidney. However, values for
GFR between 60 and 89 ml/min/1.73 m2 are mildly decreased compared to the usual values
in young healthy people. Thus it is important that clinicians appreciate that eGFR
values that are not flagged because they are >60 ml/min/1.73 m2 are not necessarily
normal.
Evidence Base
Numerous equations have been developed to estimate GFR or CrCl in adults. In general,
GFR estimating equations using creatinine include age, sex, race, and body size as
surrogates for creatinine generation by muscle. For our review of GFR estimating equations,
we only considered equations that were developing using assays that were traceable
to reference methods and study populations in which SCr concentration was measured
using traceable assays (Supplemental Table 1).
85
Based on published data, only the Modification of Diet in Renal Disease (MDRD) Study
equation, Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation and
modifications of these equations were developed using creatinine assays traceable
to the international reference material for creatinine (Table 12).
86, 87
The Cockcroft and Gault formula and others were developed before standardization of
creatinine assays but cannot be re-expressed for use with standardized creatinine
assays (Supplemental Table 2).
The MDRD Study equation was developed in 1999 and is currently recommended for eGFR
reporting in adults by the National Kidney Disease Education Program (NKDEP) and by
the Department of Health in the UK. It uses standardized SCr, age, sex, and race (black
versus white and other) to estimate GFR adjusted for BSA (ml/min/1.73 m2).
86, 94
Because of imprecision at higher GFR, NKDEP recommends that eGFR ≥60 ml/min/1.73 m2
computed using the MDRD Study equation not be reported as a numeric value. For a similar
reason, the UK Department of Health recommends not reporting eGFR >90 ml/min/1.73 m2
using the MDRD Study equation as a numeric value.
The CKD-EPI equation was developed in 2009 and uses the same four variables as the
MDRD Study equation.
87
The CKD-EPI equation had less bias than the MDRD Study equation, especially at GFR≥60 ml/min/1.73 m2,
a small improvement in precision, and greater accuracy (Figure 11). Most but not all
studies from North America, Europe and Australia show that the CKD-EPI equation is
more accurate than the MDRD Study equation, especially at higher GFR (Table 13),
85
which enables reporting of numeric values across the range of GFR. At this time, large
commercial clinical laboratories in the US have changed from using the MDRD Study
equation to the CKD-EPI equation for eGFR reporting.
Lesser bias of the CKD-EPI equation compared to the MDRD Study equation reflects higher
eGFR throughout most of the range for age and creatinine, especially in younger individuals,
women and whites. Higher eGFR results in lower prevalence estimates for CKD in these
groups (Figure 12), with more accurate risk relationships of lower eGFR and adverse
outcomes (Figure 13).
107
To account for possible differences in muscle mass and diet according to race, ethnicity
and geographic region, the MDRD Study and CKD-EPI equations have been modified for
use in other racial and ethnic groups and in other countries. In some, but not all
studies, these modifications are associated with increased accuracy (Table 14), and
should be used in preference to unmodified equations. Where tested, the CKD-EPI equation
and its modifications were generally more accurate than the MDRD Study and its modifications.
In the absence of specific modifications for race, ethnicity, or regional difference,
it is reasonable to use the CKD-EPI equation for GFR estimation. Reliance upon SCr
alone is not an appropriate alternative since the uncertainty about the effect of
non-GFR determinants affects interpretation of SCr as much as it affects interpretation
of eGFR. More widespread testing of GFR estimating equations is necessary to resolve
uncertainties about the need for racial, ethnic, and geographic modifications.
108
Pediatric Considerations
This recommendation would need to be altered for application in pediatric practice
in the following way.
Creatinine measurements in all infants and children should be derived from methods
that minimize confounders and are calibrated against an international standard.
eGFRcreat may only be reported when the height of the child is known by the laboratory.
If reporting eGFRcreat laboratories should utilize the most current and accurate pediatric
derived equations based on the demographic and laboratory markers available.
In infants or small children the level of creatinine when measured is often below
that of the normal ‘bottom range' of the adult assay. As such laboratories measuring
creatinine in infants or small children must ensure their lower calibration samples
include the lowest end of the expected range of values for the group of interest.
As the majority and the most accurate of the published pediatric eGFRcreat formulas
require height, standard laboratory reporting of eGFRcreat is neither practical nor
recommended in children. In a pediatric CKD population, and using the plasma disappearance
of iohexol as the gold standard measure of GFR, Schwartz et al. derived a number of
novel GFR prediction equations.
80
Their analysis demonstrated the importance of the height/SCr variable in the population
as it provided the best correlation with the iohexol GFR (R2=65%). The simplest of
such formula, using only height and SCr and a constant of either 41.3 or 0.413 depending
on whether height was expressed as meters or centimeters respectively, provided 79%
of estimated GFRs within 30% of the iohexol values and 37% of estimated GFRs within
10% of the iohexol values.
Any eGFRcreat formula used in children will preferably be validated at the appropriate
age and level of renal function, and the laboratory methods used locally will be calibrated
or comparable to those used in the process of developing the formula being applied.
Currently the most robust pediatric eGFR formulas, derived using iohexol disappearance
and creatinine measurements which were measured centrally and calibrated and traceable
to international standards come from the CKiDs study.
80
The two most common creatinine-based formulas recommended for use in clinical practice
include:
Updated “Bedside” Schwartz equation:
eGFR (ml/min/1.73m2)=41.3 × (height/SCr), where height is in meters and SCr is in
mg/dl.
“1B” Equation (include blood urea nitrogen [BUN] not cystatin C):
eGFR (ml/min/1.73m2)=40.7 × (height/SCr)0.64 × (30/BUN)0.202, where height is in meters,
SCr and BUN are in mg/dl.
The additional recommendation for laboratory reporting of SCr is fully applicable
in pediatrics.
When the individual clinician has information regarding current and accurate height
and applies the appropriate pediatric formula, the recommendation to report an individual
child's eGFRcreat value of less than 60 ml/min/1.73 m2 as “decreased,” would be applicable
in children over the age of 2 years.
1.4.3.5: We suggest measuring cystatin C in adults with eGFRcreat 45-59 ml/min/1.73m2
who do not have markers of kidney damage if confirmation of CKD is required. (2C)
If eGFRcys/eGFRcreat-cys is also <60 ml/min/1.73 m2, the diagnosis of CKD is confirmed.
If eGFRcys/eGFRcreat-cys is ≥60 ml/min/1.73 m2, the diagnosis of CKD is not confirmed.
RATIONALE
A major foundation of this guideline is that CKD classification and staging should
be influenced primarily by clinical prognosis. As will be reviewed in the sections
below, abundant evidence has shown that GFR estimates based on cystatin C are more
powerful predictors of clinical outcomes than creatinine-based eGFR. These findings
have been strongest for mortality and CVD events, and the prognostic advantage of
cystatin C is most apparent among individuals with GFR >45 ml/min/1.73 m2. In addition,
new findings show that using cystatin C in addition to SCr can lead to improved accuracy
of GFR estimation, including CKD classification. In the opinion of the Work Group,
these considerations warrant new recommendations for GFR estimation using cystatin
C.
Evidence Base
Evidence supports the use of cystatin C-based eGFR within the population of persons
diagnosed with CKD based on an eGFRcreat 45-59 ml/min/1.73 m2 (G3a) but without albuminuria
(A1) or other manifestations of kidney damage. This group represents 3.6% of the US
population and 41% of people in the US estimated to have CKD based on eGFRcreat and
urine ACR alone (Figure 8), and there has been substantial controversy over whether
or not these persons have CKD. Data described below indicate that use of cystatin
C to estimate GFR in this population leads to more accurate estimation of GFR and
prediction of risk for future adverse events.
In several studies, eGFRcys has been measured in populations with and without eGFRcreat
<60 ml/min/1.73 m2, and participants were separated into those with and without eGFRcys
<60 ml/min/1.73 m2 (Figure 14). Those with both eGFRcreat and eGFRcys<60 ml/min/1.73 m2,
about two-thirds of those with eGFRcreat <60 ml/min/1.73 m2, had markedly elevated
risks for death, CVD, and ESRD end points compared with persons with eGFRcreat >60 ml/min/1.73 m2.
The Work Group therefore considers this group to have “confirmed CKD.” In contrast,
about one-third of those with eGFRcreat<60 ml/min/1.73 m2 had eGFRcys>60 ml/min/1.73 m2
and this group were similar in risk for adverse outcomes as persons with eGFRcreat>60 ml/min/1.73 m2.
New data from CKD-EPI also showed improved accuracy in GFR estimation using both creatinine
and cystatin C (eGFRcreat-cys) compared to either marker alone. In the subgroup with
eGFRcreat 45-59 ml/min/1.73 m2, the combined equation correctly reclassified 16.8%
of those with eGFR 45-59 ml/min/1.73 m2 to measured GFR ≥60 ml/min/1.73 m2.
113
The consensus of the Work Group was therefore that the large group of persons with
eGFRcreat 45-59 ml/min/1.73 m2 without markers of kidney damage, but with eGFRcys/eGFRcreat-cys
≥60 ml/min/1.73 m2 could be considered not to have CKD. The removal of the diagnosis
and label of CKD may be reassuring to patients and may help clinicians to focus their
efforts on higher risk CKD patients.
The guideline statement suggesting the use of eGFRcys/eGFRcreat-cys requires several
important qualifiers. First, clinicians may not want or need to confirm the diagnosis
of CKD in patients with eGFRcreat 45-59 ml/min/1.73 m2 without markers of kidney damage,
either because the likelihood of CKD is high because of the presence of risk factors
for CKD or presence of complications of CKD. Second, cystatin C is not universally
available, so it may not be practical for a clinician to request a cystatin C blood
test. Third, in certain clinical settings, the cost of measuring cystatin C (US $1–5)
may be prohibitive. For all these reasons, the guideline statement 1.4.3.5 is stated
as a suggestion.
In addition to the population described above, eGFRcys may be useful as a confirmatory
test in situations where either the eGFRcreat may be inaccurate or biased, or when
the clinical scenario warrants a secondary test (Recommendation 1.4.3.2). In these
clinical situations, a clearance measurement using an exogenous filtration marker
may be optimal when it is available. The measurement of eGFRcys/eGFRcreat-cys would
be a relatively low-cost, feasible alternative when GFR measurement is not practical.
The Work Group believed that measured urinary CrCl was an inferior confirmatory test
relative to either GFR measurement or GFR estimation using both creatinine and cystatin
C.
If cystatin C testing is desired, it is very important that clinicians understand
principles of GFR estimation using cystatin C. As with creatinine, GFR should be estimated
from cystatin C and an appropriate equation should be chosen for the specific clinical
population (Recommendation 1.4.3.6), and an assay be chosen for measurement that is
traceable to the international standard reference material (Recommendation 1.4.3.7).
Pediatric Considerations
The utility of this specific statement to pediatrics is unclear as the vast majority
of children with significant reductions in GFR, e.g., below 60 ml/min/1.73 m2, have
either structural abnormalities or findings of renal damage as evidenced by urinary
or serum abnormalities. It is very unlikely that isolated reduction in GFR would occur
as in older adults. As such, the confirmation of CKD will be made on criteria beyond
that of GFR alone.
1.4.3.6: If cystatin C is measured, we suggest that health professionals (2C):
use a GFR estimating equation to derive GFR from serum cystatin C rather than relying
on the serum cystatin C concentration alone.
understand clinical settings in which eGFRcys and eGFRcreat-cys are less accurate.
RATIONALE
Cystatin C is licensed for use in some countries in Europe and has been approved by
the FDA as a measure of kidney function in the United States for nearly 10 years.
In certain regions, notably Sweden and parts of China, eGFR is routinely estimated
by both creatinine and cystatin C. As with creatinine, GFR estimates using cystatin
C are more accurate in estimating measured GFR than the SCysC concentration alone.
As with creatinine, sources of error in GFR estimation from SCysC concentration include
non-steady state conditions, non-GFR determinants of SCysC, measurement error at higher
GFR, and interferences with the cystatin C assays (Table 15).
Pediatric Considerations
For Recommendation 1.4.3.6, this guideline is fully applicable in pediatrics. See
Recommendation 1.4.3.7 for details.
In terms of clinical settings where eGFRcys might be less accurate, it should be noted
that Schwartz et al. determined that the only variable that explained the outlier
values of estimated GFR (in both univariate and multivariate formulas) was heavier
weight; race, high blood pressure, albumin levels and use of steroids did not contribute.
115
1.4.3.7: We recommend that clinical laboratories that measure cystatin C should (1B):
measure serum cystatin C using an assay with calibration traceable to the international
standard reference material.
report eGFR from serum cystatin C in addition to the serum cystatin C concentration
in adults and specify the equation used whenever reporting eGFRcys and eGFRcreat-cys.
report eGFRcys and eGFRcreat-cys in adults using the 2012 CKD-EPI cystatin C and 2012
CKD-EPI creatinine-cystatin C equations, respectively, or an alternative cystatin
C-based GFR estimating equations if they have been shown to improve accuracy of GFR
estimates compared to the 2012 CKD-EPI cystatin C and 2012 CKD-EPI creatinine-cystatin
C equations.
When reporting serum cystatin C:
We recommend reporting serum cystatin C concentration rounded to the nearest 100th
of a whole number when expressed as conventional units (mg/l).
When reporting eGFRcys and eGFRcreat-cys:
We recommend that eGFRcys and eGFRcreat-cys be reported and rounded to the nearest
whole number and relative to a body surface area of 1.73 m2 in adults using the units
ml/min/1.73 m2.
We recommend eGFRcys and eGFRcreat-cys levels less than 60 ml/min/1.73m2 should be
reported as “decreased.”
RATIONALE
As for SCr, reporting eGFR using cystatin C in addition to cystatin C will facilitate
clinician's use of cystatin C for GFR estimation. It is important to acknowledge that
calibration of assays is essential to interpretation of kidney function measures.
Cystatin C is measured by a variety of immunoassays and, as for creatinine, there
can be variation among methods in reported SCysC concentration but reported analytic
variation appears less common than with creatinine. In June 2010 the Institute for
Reference Materials and Measurements (IRMM) released a reference material (ERM-DA471/IFCC)
for cystatin C measurement. Reagent manufacturers are in the process of recalibrating
their assays against this standard which will enable standardized reporting of cystatin
C and eGFR results. This recommendation is directed to laboratories with the intent
to clarify the details of such calibration and the use of specific equations so as
to facilitate international standardization.
Evidence Base
Numerous equations have been developed to estimate GFR. Some equations include cystatin
C as the only variable, while others include, age, sex, or race, but the magnitude
of coefficients for these variables are smaller than in creatinine-based equations,
presumably reflecting less contribution of muscle to cystatin C generation than to
creatinine generation. Equations without race are a potential advantage for cystatin
C-based estimating equations in non-black, non-white populations.
For our review of GFR estimating equations, we only considered equations that were
developed using assays that were traceable to the new reference methods and study
populations in which SCysC concentration was measured using traceable assays. At this
time, only the equations developed by CKD-EPI are expressed for use with standardized
SCysC (Table 16), including equations developed in CKD populations in 2008
116, 117
and re-expressed for use with standardized cystatin C in 2011, and equations developed
in diverse populations in 2012.
113
Equations using assays that are not traceable to the the reference standard are listed
in Supplemental Table 3.
The 2012 creatinine-cystatin C equation is more accurate than equations using creatinine
or cystatin C separately (Figure 15), and more accurate than the 2008 creatinine-cystatin
C equation (Table 17). The average of the GFR computed by the equations using creatinine
and cystatin C separately is similar to the GFR computing using the creatinine-cystatin
C equations. The 2012 cystatin C equation has similar accuracy to the 2009 creatinine
equation described above but does not require use of race, and may be more accurate
in non-black, non-white populations or in clinical conditions with variation in non-GFR
determinants of SCr. We anticipate the development of additional equations using cystatin
C in the future and recommend that they be compared with the CKD-EPI 2012 cystatin
C and creatinine-cystatin C equations as well as with the CKD-EPI 2009 creatinine
equation.
Pediatric Considerations
For Recommendation 1.4.3.7 this set of statements would need to be altered for application
in pediatric practice in the following way:
Measure SCysC using an immunonephelometrically determined method in which the assay
is calibrated and traceable to the international standard reference material.
Report eGFRcys in addition to the SCysC concentration in children.
Report eGFRcys in children specifying the specific equation used.
Based on their recent work comparing particle-enhanced nephelometric to turbidometric
immunoassays for cystatin C in a pediatric population with significant reduction in
GFR (median GFR ∼45 ml/min/1.73 m2), Schwartz et al. demonstrated less bias for the
nepholometric value and that its reciprocal showed a substantially improved correlation
to the iohexol GFR (0.87 versus 0.74) when compared to that of the turbidometric assay.
115
This demonstrates the importance of using an assay calibrated and traceable to the
international standard reference material.
Numerous pediatric specific and derived eGFRcys formulas have been published, the
most current and recent by Schwartz
115
who derives the newest available from a validation set from the CKiD study and compares
those results to 3 other well-recognized formulas from the literature, namely Zapitelli
et al.,
118
Filler and Lepage,
119
and Hoek et al.
120
Their results demonstrate that the newest univariate cystatin C formula derived from
the CKiD cohort has excellent accuracy with 82.6% of eGFRcys within 30% of the true
measured iohexol GFR and 37.6% within 10% of the true measured iohexol GFR. Likewise
the bias of 0.3% and correlation of 0.85 are the best of all formulas reported to
date. The formula they use to obtain these values is: 70.69 × (cystatin c)-0.931.
Of note, their final multivariate equation, when applied to the validation set and
using height/SCr, nepholemetric cystatin C, BUN, sex, and an adjusted height term
demonstrated the best accuracy reported in pediatric studies to date, 91% and 45%
within 30% and 10% of the true GFR, respectively; with a bias of only −0.2 and correlation
of 0.92.
1.4.3.8: We suggest measuring GFR using an exogenous filtration marker under circumstances
where more accurate ascertainment of GFR will impact on treatment decisions. (2B)
RATIONALE
In clinical practice, there may be a requirement to measure GFR when the need for
a ‘truer' more precise value is identified (such as for organ donation or for dosing
of toxic drugs). The intention of this statement is to recognize that specialty centers
for kidney disease, usually tertiary referral centers, should have the capacity to
measure GFR using exogenous filtration markers as a recognized specialist service.
We recognize that this ability does not currently constitute the definition of specialty
kidney referral centers and that it may be problematic, but resources to ensure accurate
measurement ought to be made available. Given that these specific measurements require
levels of rigor and reproducibility similar to those of laboratory calibration issues,
specialist centers would be the right place to suggest that these facilities be made
available.
Evidence Base
GFR is measured as the clearance of an exogenous filtration marker. The “gold standard”
method is the urinary clearance of inulin during a continuous intravenous infusion.
To simplify the procedure there are a number of alternative clearance methods and
alternative filtration markers, with minor differences among them.
79
For all measurement methods, measured GFR should be reported as described for eGFR.
Table 18 summarizes the strengths and limitations of clearance methods and filtration
markers for clearance measurements. Thus measured GFR may also be associated with
error, and in evaluation of GFR estimating equations, random error in GFR measurement
is a source of some of the imprecision in GFR estimating equations.
27, 121
In principle, the magnitude of random error in GFR measurements is likely to be smaller
than errors in GFR estimation using creatinine and cystatin C due to conditions listed
in Tables 11 and 15.
International Relevance
The calculation of eGFR using these equations usually requires computer programming
and some processes for quality monitoring. Nonetheless the statements are here to
serve as ‘best practice' recommendations so that these can be aspired to over time
in those locations where these recommendations are currently not able to be implemented.
The Work Group appreciated that not all laboratories have capabilities to assay cystatin
C. Different countries and regions will have different availabilities for measurement
of GFR. The statement about GFR measurements mostly applies to countries with tertiary
care services such as kidney transplantation and oncology.
Implications for Clinical Practice and Public Policy
It is important for clinicians to understand various methods for estimating and measuring
kidney function and the situations in which specific methods may be superior in clinical
decision making about treatment and referral.
Standardized assays and robust equations are important for epidemiological and planning
purposes so that public policy can be informed by more accurate estimates of CKD,
which may be possible with improved standardization of both assays and equations.
In different parts of the world, different assays are used and equations for estimating
eGFR may differ. Thus, appreciating and understanding local standards is important
for individual patients who may travel, and for comparative research across countries
or regions.
In the event that a clinician requires measurement of GFR instead of an estimate,
knowledge of these different tests and availability of them is important. Situations
in which measurement would be required are likely quite infrequent but include donor
evaluation in kidney transplantation and use of toxic drugs which have a narrow therapeutic
range. We acknowledge that drug development and clinical observation programs may
not define the various thresholds with sufficient granularity to require greater accuracy
than is provided by eGFRcreat. Guidance is evolving regarding kidney function evaluation
during drug development programs.13 There are no direct implications for public policy
for the statement about GFR measurement.
Areas of Controversy, Confusion, or Non-consensus
The Work Group recognizes that no single creatinine-based estimating equation will
perform optimally in all clinical circumstances and that there may be changes in the
performance of estimating equations over time and in different regions. However, for
the purpose of eGFR reporting, it is important to select a single equation within
a region or country. At the writing of this guideline, in North America, Europe, and
Australia, the advantages of the CKD-EPI equation at higher GFR make it more applicable
than the MDRD Study equation for general practice and public health.
While cystatin C offers some advantages over SCr as the basis of estimating equations,
the cost of the assay and potential lack of standardization across laboratories for
this ‘newer' test limit our ability to recommend it as a preferred or even usual second
test after creatinine. We recognize that these factors may lead to variations in implementation.
The recommendation to consider confirmatory or additional testing if there is a need
for more accurate determination of GFR is important. That there are other laboratory
markers to estimate GFR (i.e., cystatin C) is stated here as there has been accumulating
data to support its use in these situations. We have specifically mentioned cystatin
C because of these data.
Clarification of Issues and Key Points
It is important for clinicians to appreciate the need for standardized assays and
standardized equations for laboratory reporting of eGFR. Changes in laboratory assays
or calculation methods should be reported to clinicians in order to avoid confusion
when serially following individuals. This is because values in an individual might
indicate a worsening or improvement in eGFR which may be attributable to different
assays or calculation methods, rather than a reflection of true change.
When precise information about GFR is required, direct measurement using reliable
methods should be pursued.
Pediatric Considerations
For Recommendation 1.4.3.8 this guideline is fully applicable in pediatrics.
1.4.4
Evaluation of albuminuria
1.4.4.1: We suggest using the following measurements for initial testing of proteinuria
(in descending order of preference, in all cases an early morning urine sample is
preferred) (2B):
urine albumin-to-creatinine ratio (ACR);
urine protein-to-creatinine ratio (PCR);
reagent strip urinalysis for total protein with automated reading;
reagent strip urinalysis for total protein with manual reading.
1.4.4.2: We recommend that clinical laboratories report ACR and PCR in untimed urine
samples in addition to albumin concentration or proteinuria concentrations rather
than the concentrations alone. (1B)
1.4.4.2.1: The term microalbuminuria should no longer be used by laboratories. (Not
Graded)
1.4.4.3: Clinicians need to understand settings that may affect interpretation of
measurements of albuminuria and order confirmatory tests as indicated (Not Graded):
Confirm reagent strip positive albuminuria and proteinuria by quantitative laboratory
measurement and express as a ratio to creatinine wherever possible.
Confirm ACR ≥30 mg/g (≥3 mg/mmol) on a random untimed urine with a subsequent early
morning urine sample.
If a more accurate estimate of albuminuria or total proteinuria is required, measure
albumin excretion rate or total protein excretion rate in a timed urine sample.
RATIONALE
We recommend measurement of urinary albumin because it is relatively standardized
and because it is the single most important protein lost in the urine in most chronic
kidney diseases. Use of urinary albumin measurement as the preferred test for proteinuria
detection will improve the sensitivity, quality, and consistency of approach to the
early detection and management of kidney disease.
By contrast, laboratory tests purporting to measure urinary total protein are commonly
flawed, often being standardized against, and predominantly sensitive to, albumin.
They have poor precision at low concentrations and demonstrate poor between-laboratory
agreement while being insensitive, non-specific, and susceptible to a range of false-positive
and false-negative problems. There may occasionally be clinical reasons for a specialist
to use PCR instead of ACR to quantify and monitor significant levels of proteinuria
(e.g., in patients with monoclonal gammopathies).
Commonly used reagent strip devices measuring total protein are insufficiently sensitive
for the reliable detection of proteinuria, do not adjust for urinary concentration,
and are only semi-quantitative. Furthermore, there is no standardization between manufacturers.
The use of such strips should be discouraged in favor of quantitative laboratory measurements
of albuminuria or proteinuria. When used, reagent strip results should be confirmed
by laboratory testing (Figure 16).
The combination of reagent strips with automated reader devices can improve inter-operator
variability. More recently launched reagent strip devices capable of producing albumin
or total protein results as a ratio to urinary creatinine require further evaluation
to provide evidence that they have equivalent sensitivity and specificity to laboratory
tests and are economically advantageous.
Although the reference point remains the accurately timed 24-hour specimen, it is
widely accepted that this is a difficult procedure to control effectively and that
inaccuracies in urinary collection may contribute to errors in estimation of protein
losses. In practice, untimed urine samples are a reasonable first test for ascertainment
of albuminuria. An EMU (‘first pass') sample is preferred since it correlates well
with 24-hour protein excretion, has relatively low intra-individual variability, and
is required to exclude the diagnosis of orthostatic (postural) proteinuria. However,
a random urine sample is acceptable if no EMU sample is available.
The concentration of protein or albumin in a urine sample will be affected by hydration
(i.e., how diluted or concentrated a urine sample is). Creatinine excretion is considered
to be fairly constant throughout the day and it has become customary to correct for
urinary concentration by expressing either the protein or albumin concentrations as
a ratio to the creatinine concentration in the same sample.
Timed urine collections may be used for confirmatory purposes but are not required
except in circumstances in which untimed urine ACR is less accurate. It is worthwhile
noting that albumin and protein excretion display considerable biological variability
and may be increased by a variety of pathological and non-pathological factors. Consequently,
confirmation of increased excretion rates is recommended.
Evidence Base
Why is albumin measurement being recommended instead of total protein?
Urine albumin measurement provides a more specific and sensitive measure of changes
in glomerular permeability than urinary total protein.
123, 124, 125
There is substantial evidence linking increased albuminuria to outcomes of CKD
4, 30
(e.g., CKD Prognosis Consortium
2, 3, 4, 5
, Nord-Trøndelag Health Study [HUNT 2]
125a
, Prevention of Renal and Vascular Endstage Disease [PREVEND]
125b
). There is also evidence that urinary albumin is a more sensitive test to enable
detection of glomerular pathology associated with some other systemic diseases including
diabetes, hypertension and systemic sclerosis.
126, 127, 128, 129
In health, relatively small amounts of albumin (<30 mg/24 hours) are lost in the urine.
Because of this, and additionally because total protein assays are imprecise and insensitive
at low concentrations, relatively large increases in urine albumin excretion can occur
without causing a significant measurable increase in urinary total protein.
125
Total protein measurement is problematic in urine due to: large sample-to-sample variation
in the amount and composition of proteins; high and variable concentrations of non-protein
interfering substances relative to the protein concentration; and high inorganic ion
content. All these factors affect the precision and accuracy of the various methods.
Most laboratories currently use either turbidimetry or colorimetry
130
to measure total protein and as with urine reagent strip analysis, these methods do
not give equal analytical specificity and sensitivity for all proteins which can contribute
to diverse estimates of proteinuria prevalence.
131, 132
Most methods tend to react more strongly with albumin than with globulin and other
non-albumin proteins.
34, 133, 134, 135
There are significant interferences causing falsely high results.
136, 137, 138
. There is no reference measurement procedure and no standardized reference material
for urinary total protein listed by the JCTLM. The variety of methods and calibrants
in use means that there is inevitably significant between-laboratory variation.
139, 140, 141
Since a variable mixture of proteins is measured, it is difficult to define a standardized
reference material.
How should albumin be measured and reported?
Albumin should be measured using immunological assays capable of specifically and
precisely quantifying albumin at low concentrations and of producing quantitative
results over the clinically relevant range. Currently urinary albumin is predominantly
measured by diagnostic laboratories using turbidimetric assays.
130
At present there is no reference measurement procedure or standardized reference material
for urine albumin listed by the JCTLM, although the NKDEP and the International Federation
of Clinical Chemistry and Laboratory Medicine have recently established a joint committee
to address these issues.
142, 143
At present, most assays are standardized against a serum-based calibrant (CRM 470)
distributed by the IRMM of the European Commission, as has previously been recommended
by KDIGO.
31
Albumin concentration should be reported as a ratio to urinary creatinine concentration
(mg/mmol or mg/g). ACR results should be expressed to one decimal place (mg/mmol)
or whole numbers (mg/g). Both enzymatic and Jaffe assays are suitable for the measurement
of creatinine in urine. We suggest that the term ‘microalbuminuria' no longer be used
because it can be misleading in suggesting that the albumin may be small or different
in some way. The proposed albuminuria categories A1-3 are a more clinically meaningful
way to express information about categories within the continuum of albumin excretion.
Reagent strip point-of-care testing devices capable of measuring low concentrations
of albumin are also available producing both semi-quantitative and fully quantitative
ACR results. Reasonable analytical
144, 145, 146, 147
and diagnostic performance has been demonstrated.
148, 149, 150
While studies of these devices have been somewhat limited in size, they demonstrate
their potential to play a significant role in the care pathway of patients suspected
of having CKD.
Why are reagent strip devices for protein measurement considered less accurate than
laboratory measurement?
Reagent strip devices for proteinuria detection have been in use for more than 50
years. As discussed earlier, a positive reagent strip result is also associated with
outcomes of CKD. Such devices have been used to support screening programs in some
countries,
151, 152, 153
although there appears to be no evidence supporting such screening of unselected populations.
154
Although purporting to measure total protein, the reagent pad is most sensitive to
albumin.
155, 156, 157
There is evidence that strips from different manufacturers perform differently at
the cutoff (‘+') concentration of 300 mg/l and degrees of ‘plus-ness' between different
manufacturers don't always correspond to the same nominal concentration of protein
in urine.
124
Concentrated urines may give a color change in the positive range of a reagent strip
device even though protein loss remains normal and vice versa. False-positive results
may occur if the urine is alkalinized (e.g., due to urinary tract infection) or in
the presence of quaternary ammonium compounds that alter the pH of the urine. The
performance of reagent strips is operator-dependent
158
and affected by the presence of colored compounds such as bilirubin and certain drugs
(e.g., ciprofloxacin, quinine, and chloroquine).
159
Reagent strips cannot reliably distinguish between proteinuria categories
124, 157
and show relatively poor diagnostic accuracy for proteinuria detection.
160, 161
In the Australian Diabetes, Obesity and Lifestyle (AusDiab) study, a reagent strip
reading of + or greater had 58% and 99% sensitivity for detecting ACR ≥30 mg/g (≥
3 mg/mmol) and ≥300 mg/g (≥30 mg/mmol), respectively. 47% of individuals who tested
+ or greater had an ACR ≥30 mg/g (≥ 3 mg/mmol) on laboratory testing.
162
Automated devices capable of reading the color changes of reagent strips using reflectance
spectrometry are available. These reduce inter-operator variability and improve diagnostic
accuracy.
150, 158, 163
A creatinine test pad has been added to some reagent strip systems to enable a PCR
to be reported and so reduce the intra-individual variation seen with random urine
collections. Such devices have been shown to be suitable for ruling out significant
proteinuria (>300 mg/24 hours) in an outpatient setting.
149
Correcting for urinary dilution.
Since creatinine excretion in the urine is fairly constant throughout the 24-hour
period, measurement of ACR (or PCR) allows correction for variations in urinary concentration.
164, 165
ACR is a suitable alternative to timed measurement of urine albumin loss.
143, 166, 167, 168, 169, 170
PCR on random or early morning untimed samples shows good diagnostic performance and
correlation with 24-hour collection.
160, 163, 171, 172, 173, 174, 175, 176, 177
Expressing albumin as a ratio to creatinine reduces intra-individual variability:
lowest variability for the ACR has been reported in EMU samples as opposed to other
untimed samples or timed collections.
142, 178
In one study albumin variability was reduced from 80% to 52% when expressed as an
ACR rather than an albumin concentration.
179
The within-subject biological variation for urinary ACR in an EMU has been reported
to be 31%, compared to 36% for urinary albumin concentration.
180
The same study reported variability for ACR of 103% and 85% in random and timed 24-hour
collections, respectively.
180
Intra-individual variation for protein loss is also significantly reduced when reported
as a PCR compared to protein concentration in random urine samples collected throughout
the day (a mean reduction from 97% to 39%).
179
Why and how should a finding of albuminuria be confirmed?
Given the high biological variation and other pathological and physiological causes
of albuminuria (Table 19),
143
repeat testing to confirm albuminuria, ideally using an EMU and laboratory testing,
is recommended (Figure 16).
There has been extensive discussion in the literature about the appropriate urine
sample to use for the investigation of protein loss. It is generally recognized that
a 24-hour sample is the definitive means of demonstrating the presence of proteinuria.
However, overnight, first void in the morning (i.e., EMU), second void in the morning,
or random sample collections can also be used. In a systematic review random urine
PCR was shown to have better performance as a test for ruling out significant proteinuria
than as a “rule-in” test; the authors suggested that positive PCR results may still
require confirmation with a 24-hour collection.
185
If an EMU is unavailable, subsequent samples can give a reliable indication of the
24-hour urine protein loss.
174
International Relevance
The recommendation to replace urinary total protein with albumin as the test of choice
in testing for proteinuria is consistent with most,1,
31, 130, 186, 187
but not all,
188, 189
current national and international guidance. It is accepted that cost pressures may
affect implementation of this recommendation and may differ across the world.
Most international guidelines have also discouraged the use of reagent strip analysis
for proteinuria detection.
186, 189, 190, 191
Nevertheless, in the present guideline we acknowledge that these devices may have
a role, particularly in settings where access to laboratory services may be limited.
ACRs in North America tend to be reported in mg/g whereas in other parts of the world
usage of mg/mmol predominates. This difference appears unlikely to be resolved in
the foreseeable future. When publishing data authors should ensure either that both
units are cited or that a conversion factor is provided.
There is increasing adoption of the term ‘albuminuria' instead of microalbuminuria
by international and national laboratory and some clinical organizations.
Implications for Clinical Practice and Public Policy
Direct reagent costs of total protein measurement are generally lower than those of
albumin measurement, which requires antibody-based reagents. It is often considered
that reagent strip analyses are a cheaper option. Therefore some health-care systems
may struggle to justify the recommendations in this guideline.
Costs of diagnostic tests vary depending on local financial agreements between hospitals
and suppliers. In England, the National Institute for Health and Clinical Excellence
(NICE) sampled a small random number of laboratories and estimated the average cost
of an ACR to be £2.16 whereas a PCR cost £1.42.
186
It is acknowledged that increased use of ACR testing may reduce the unit cost on the
basis of economies of scale. In Canada, laboratory analysis costs (Canadian dollars)
of $2.81 for reagent strip, $11.67 for PCR, and $29.23 for ACR have been cited.
192
In relation to albumin-specific reagent strips, a cost of approximately $4 for a Micral
test II (Roche Diagnostics) compared to $2 for a laboratory ACR has been reported.
193
The cost- and clinical-effectiveness of an approach utilizing reagent strip testing
followed by laboratory measurement compared to an approach in which samples are submitted
directly to the laboratory (for either albumin or protein measurement) has recently
been evaluated in a health economics model.
186
The model favored abandoning the use of reagent strips for identification of proteinuria.
Areas of Controversy, Confusion, or Non-consensus
Some data suggest that ACR is a poorer predictor of 24-hour total protein loss than
PCR
194
and has no advantage over PCR as a predictor of renal outcomes and mortality in patients
with CKD.
195, 196
In the prediction of future transplant rejection, PCR has been reported to have equal
utility to ACR,
192
although in a separate study ACR was found to be a better predictor.
197
In the setting of preeclampsia, proteinuria is generally defined as ≥300 mg/24 hours
or a PCR ≥300 mg/g (≥30 mg/mmol).
175
Currently, there is insufficient evidence to substitute urine albumin measurement
for total protein in this setting.
172
Creatinine excretion is affected by a variety of non-renal influences (Table 19) and
it therefore follows that different cutoffs for ACR (and PCR) may be required in different
individuals.
194, 198
While age-related cutoffs have not generally been applied in clinical practice, clinicians
should bear this in mind when interpreting urine ACR data in older individuals or
those with very low body mass, as these will impact the urine creatinine excretion.
While most guidelines agree that an ACR greater than approximately 3 mg/mmol (30 mg/g)
is pathological in the setting of diabetes, in the non-diabetic population a higher
threshold has commonly been used to define proteinuria. In the NICE guideline in England
and Wales, proteinuria in non-diabetic individuals was defined as ≥30 mg/mmol (≥300 mg/g),
with higher level proteinuria being >70 mg/mmol (>700 mg/g).
186
Confirmation of results lying between 30 and 70 mg/mmol (300-700 mg/g) was recommended.
186
The present guideline proposes a lower threshold definition for albuminuria for use
in both diabetic and non-diabetic individuals.
A study from Italy in type 2 diabetes has reported that, although intra-individual
biological variation of albuminuria is large, a single sample (either ACR or timed
collection) can accurately classify patients into albuminuria categories, negating
the need for multiple collections.
178
Some data suggest that a significant proportion of albumin present in urine may be
non-immunoreactive,
199, 200, 201, 202
although this finding has been questioned.
203, 204
There is a substantial existing literature using the term microalbuminuria and many
existing guidelines use this term especially in the context of diabetes and cardiovascular
risk, as its presence confers risk. Nonetheless, the Work Group believes that it is
important for this international guideline to foster ‘best practices' and clarity
of communication, and since the risk of adverse events is continuous throughout the
spectrum of albuminuria, we encourage adoption of the term ‘albuminuria' with subsequent
quantification of the level or amount.
Pediatric Considerations
For Recommendation 1.4.4.1, this set of statements would need to be altered for application
in the pediatric practice as follows:
We suggest using the following measurements for initial testing of proteinuria in
children (in descending order of preference):
urine PCR, EMU sample preferred;
urine ACR, EMU sample preferred;
reagent strip urinalysis for total protein with automated reading;
reagent strip urinalysis for total protein with manual reading.
For Recommendations 1.4.4.2 and 1.4.4.3, this set of statements would need to be altered
for application in the pediatric practice as follows:
Currently the urinary PCR should be favored over the urine ACR in children. Unlike
in adults where powerful evidence exists in support of the use of measures of albumin
rather than total protein to predict adverse outcomes, this level of evidence is currently
lacking in children.
205
However, current longitudinal trials such as CKiD
55
and European 4C
78
may eventually shed light on this issue.
In children the underlying conditions associated with the diagnosis of CKD are also
important considerations as to which form of testing is most valuable. Unlike adults
where the majority of patients with CKD are attributed to an underlying glomerular
disease or hypertensive damage, the vast majority of children have underlying developmental
abnormalities often referred to as CAKUT (congenital anomalies of the kidney and urinary
tract).
70
This relative paucity of glomerular conditions makes the use of albumin excretion
a less sensitive test for diagnostic purposes as many children will have underlying
tubular conditions and hence tend to excrete more Tamm-Horsfall protein and other
low-molecular-weight proteins that will not be captured by the albumin-to-creatinine
(or formal albumin excretion) assay.
For Recommendations 1.4.4.2 and 1.4.4.2.1 this guideline is fully applicable in pediatrics.
The recommendation that clinical laboratories report ACR and PCR in untimed urine
samples in addition to albumin concentration or proteinuria concentrations rather
than the concentrations alone is valid and useful in the pediatric population. As
per Recommendation 1.2.4, however, note should be made that age-related normal values
for urinary protein losses must be considered when laboratories choose to report either
ACR or PCR.
Albuminuria in children, whether measured as an absolute value per day, an excretion
rate, or as an albumin to creatinine ratio is fraught with more uncertainity than
in adults as they are known to vary across categories of age, sex, height, weight,
and Tanner staging.
206
In two recent reviews by Rademacher
206
and Tsioufis et al.,
205
both groups examined the results of all relevant studies on normative values of AER
or ACR. Rademacher's paper in particular provides detailed information on the mean
AER values (with SD) across a variety of studies, ages, sex, and race, and provides
a normative estimate for overnight AER of between 2-6 μg/minute or a 95th percentile
value from 4.5–28 μg/minute. Similarly, they summarize results for ACR in normal children
and suggest that the mean for children older than 6 years would seem to fall between
8-10 mg/g (0.8-1.0 mg/mmol).
For Recommendation 1.4.4.3, this guideline is fully applicable in pediatrics.
1.4.4.4: If significant non-albumin proteinuria is suspected, use assays for specific
urine proteins (e.g., α1-microglobulin, monoclonal heavy or light chains, [known in
some countries as “Bence Jones”proteins]). (Not Graded)
RATIONALE
Testing for tubular proteinuria using a total protein approach almost certainly has
very poor sensitivity for detecting tubular disease. When an isolated tubular lesion
is suspected (Table 3), this is probably best investigated by measuring a specific
tubular protein (e.g., α1-microglobulin) using an immunoassay approach.
Evidence Base
There have been concerns that replacing urinary total protein measurement with albumin
measurement may cause non-albuminuric (effectively tubular and overproduction) proteinuria
to be missed. Low-molecular-weight proteinuria is a defining feature in some uncommon
kidney diseases (e.g., Dent's disease).
207
However, for some of the reasons already discussed, total protein assays will also
be poor at detecting tubular proteinuria. When investigating patients for tubular
proteinuria, it is advisable to use assays targeted at specific tubular proteins.
In the AusDiab study, of those with proteinuria (2.4% of the general population, defined
as a PCR >23 mg/mmol [230 mg/g]) 92% had albuminuria (defined as an ACR >3.4 mg/mmol
[34 mg/g]); 8% had an ACR within the reference range.
208
These individuals were less likely to have diabetes than those with both proteinuria
and albuminuria, but no further information is available as to the nature of the proteinuria
in these individuals or its likely significance. The authors speculate that these
individuals could have had light chain proteinuria or interstitial nephropathies.
Using albuminuria testing to identify proteinuria had a specificity of 95%. The negative
predictive value was 99.8% and the positive predictive value was 32.4%. The authors
concluded that testing for albuminuria rather than proteinuria was supported.
As discussed above, quite significant increases in urinary albumin loss have to occur
before such an increase is detectable on the background of a total protein assay.
The situation is even more extreme for tubular proteins which, in health, are present
in urine at lower concentrations than albumin (e.g., normal daily losses of retinol
binding protein, α1-microglobulin and β2-microglobulin are 0.08, 3.6, and 0.1 mg/d,
respectively).
209
This problem will be exacerbated by the fact that the recognition of tubular proteins
by some total protein assays is poor.
210
In disease states concentrations of tubular proteins, at least collectively, can reach
levels detectable by total protein assays. For example, among patients with tubulointerstitial
disease but without renal insufficiency, median concentrations of α1-microglobulin
were 37 mg/l, with concentrations up to 100 mg/l being observed; higher concentrations
were seen in patients with decreased GFR.
211
Among a group of patients with acute tubular necrosis requiring dialysis treatment,
median α1-microglobulin concentration was 35 mg/mmol of creatinine.
212
However, although tubular proteinuria is characterized by a relative increase in low-molecular-weight
protein concentrations, generally albumin still remains a significant component of
the total protein concentration. Indeed, it is thought that tubular disease results
in an increase in albumin loss as a result of decreased tubular reabsorption of filtered
albumin. For example, it has been estimated that when tubular absorption fails completely,
β2-microglobulin loss increases to 180 mg/24 hours (approximately 1800-fold normal)
but there will also be an increase in urinary albumin loss to about 360 mg/24 hours
(approximately 20-fold normal).
209
In a series of patients with Dent's disease, a classical tubular disorder, 21 of the
23 patients demonstrating increased urinary α1-microglobulin and β2-microglobulin
loss also had increased urinary albumin loss: those who did not had borderline increases
in tubular protein losses that would not have been detectable using a total protein
measurement approach.
207
The authors comment that in those patients in whom proteinuria was marked (>1 g/d),
urinary albumin loss was also markedly increased. In some situations, however, tubular
proteinuria in the absence of albuminuria has been reported (e.g., in some children
with type 1 diabetes
213
and in kidney scarring in reflux nephropathy
214
).
International Relevance
There is no reason to believe that there are significant differences around the world
with respect to incidence or prevalence of conditions in which measurement of non-albumin
proteins would be required. The availability of reliable tests for these alternative
proteins, however, may be different in different regions.
Implications for Clinical Practice and Public Policy
The incidence and prevalence of tubular disorders will vary geographically with the
clinical setting (e.g., adult or pediatric practice) and factors such as occupational
exposure. Clinicians should agree with their local laboratories a suitable approach
to the detection of tubular proteinuria and laboratories should be able to advise
on suitable sample handling procedures. It is acknowledged that many laboratories
do not currently offer assays of tubular proteins.
In patients with suspected myeloma, monoclonal heavy or light chains (known in some
countries as Bence Jones) protein should be sought in concentrated urine using electrophoresis
with immunofixation of any identified protein bands in accordance with current myeloma
guidelines.
215
Simultaneous albumin measurement is needed when the possibility of immunoglobulin
light chain (AL) amyloid or light chain deposition disease is suspected.
Non-albumin proteinuria may also be suspected in patients with disorders of tubular
function (see Table 3).
Areas of Controversy, Confusion, or Non-consensus
Testing for proteinuria using a urine albumin rather than total protein first-line
approach may occasionally miss cases of tubular proteinuria but the significance of
this problem is probably overestimated and should be the subject of further research.
Earlier guidance from KDOQI1 suggested that proteinuria in children should be detected
with total protein rather than albumin assays due to the higher prevalence of non-glomerular
diseases in this group of patients. For the reasons outlined above, we do not think
total protein assays are suitable for this purpose and would ideally recommend testing
for albumin and for specific tubular proteins when non-glomerular disease is suspected.
Pediatric Considerations
For Recommendation 1.4.4.4, this statement is fully applicable in pediatrics. In children
the likelihood of any form of overflow proteinuria such as seen in conditions of heavy
or light chain production is extremely low; however a significant number of underlying
genetic tubular disorders do exist and protein electrophoresis can assist the practitioner
in determining the presence of such a condition or the concurrent finding of severe
tubular injury in addition to a glomerular condition.
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