Editorial: Endocrine abnormalities and renal complications

Summary Background Health system planning requires careful assessment of chronic kidney disease (CKD) epidemiology, but data for morbidity and mortality of this disease are scarce or non-existent in many countries. We estimated the global, regional, and national burden of CKD, as well as the burden of cardiovascular disease and gout attributable to impaired kidney function, for the Global Burden of Diseases, Injuries, and Risk Factors Study 2017. We use the term CKD to refer to the morbidity and mortality that can be directly attributed to all stages of CKD, and we use the term impaired kidney function to refer to the additional risk of CKD from cardiovascular disease and gout. Methods The main data sources we used were published literature, vital registration systems, end-stage kidney disease registries, and household surveys. Estimates of CKD burden were produced using a Cause of Death Ensemble model and a Bayesian meta-regression analytical tool, and included incidence, prevalence, years lived with disability, mortality, years of life lost, and disability-adjusted life-years (DALYs). A comparative risk assessment approach was used to estimate the proportion of cardiovascular diseases and gout burden attributable to impaired kidney function. Findings Globally, in 2017, 1·2 million (95% uncertainty interval [UI] 1·2 to 1·3) people died from CKD. The global all-age mortality rate from CKD increased 41·5% (95% UI 35·2 to 46·5) between 1990 and 2017, although there was no significant change in the age-standardised mortality rate (2·8%, −1·5 to 6·3). In 2017, 697·5 million (95% UI 649·2 to 752·0) cases of all-stage CKD were recorded, for a global prevalence of 9·1% (8·5 to 9·8). The global all-age prevalence of CKD increased 29·3% (95% UI 26·4 to 32·6) since 1990, whereas the age-standardised prevalence remained stable (1·2%, −1·1 to 3·5). CKD resulted in 35·8 million (95% UI 33·7 to 38·0) DALYs in 2017, with diabetic nephropathy accounting for almost a third of DALYs. Most of the burden of CKD was concentrated in the three lowest quintiles of Socio-demographic Index (SDI). In several regions, particularly Oceania, sub-Saharan Africa, and Latin America, the burden of CKD was much higher than expected for the level of development, whereas the disease burden in western, eastern, and central sub-Saharan Africa, east Asia, south Asia, central and eastern Europe, Australasia, and western Europe was lower than expected. 1·4 million (95% UI 1·2 to 1·6) cardiovascular disease-related deaths and 25·3 million (22·2 to 28·9) cardiovascular disease DALYs were attributable to impaired kidney function. Interpretation Kidney disease has a major effect on global health, both as a direct cause of global morbidity and mortality and as an important risk factor for cardiovascular disease. CKD is largely preventable and treatable and deserves greater attention in global health policy decision making, particularly in locations with low and middle SDI. Funding Bill & Melinda Gates Foundation.

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 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 3 months to indicate CKD. A GFR 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 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 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 3 months does not apply to newborns or infants ≤3 months of age. the criteria of a GFR 1 but 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 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 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., 900 mg/g (>90 mg/mmol); slope +0.16±3.64 and −0.54±3.67 versus −3.61 ±5.47 (P 10 ml/min/1.73 m2 and found a risk ratio of 4.01 (95% CI 2.23–7.25; P 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 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 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 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. 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 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 ( 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. DISCLAIMER While every effort is made by the publishers, editorial board, and ISN to see that no inaccurate or misleading data, opinion or statement appears in this Journal, they wish to make it clear that the data and opinions appearing in the articles and advertisements herein are the responsibility of the contributor, copyright holder, or advertiser concerned. Accordingly, the publishers and the ISN, the editorial board and their respective employers, office and agents accept no liability whatsoever for the consequences of any such inaccurate or misleading data, opinion or statement. While every effort is made to ensure that drug doses and other quantities are presented accurately, readers are advised that new methods and techniques involving drug usage, and described within this Journal, should only be followed in conjunction with the drug manufacturer's own published literature.

Chapter 2.1: Definition and classification of AKI INTRODUCTION AKI is one of a number of conditions that affect kidney structure and function. AKI is defined by an abrupt decrease in kidney function that includes, but is not limited to, ARF. It is a broad clinical syndrome encompassing various etiologies, including specific kidney diseases (e.g., acute interstitial nephritis, acute glomerular and vasculitic renal diseases); non-specific conditions (e.g, ischemia, toxic injury); as well as extrarenal pathology (e.g., prerenal azotemia, and acute postrenal obstructive nephropathy)—see Chapters 2.2 and 2.3 for further discussion. More than one of these conditions may coexist in the same patient and, more importantly, epidemiological evidence supports the notion that even mild, reversible AKI has important clinical consequences, including increased risk of death. 2, 5 Thus, AKI can be thought of more like acute lung injury or acute coronary syndrome. Furthermore, because the manifestations and clinical consequences of AKI can be quite similar (even indistinguishable) regardless of whether the etiology is predominantly within the kidney or predominantly from outside stresses on the kidney, the syndrome of AKI encompasses both direct injury to the kidney as well as acute impairment of function. Since treatments of AKI are dependent to a large degree on the underlying etiology, this guideline will focus on specific diagnostic approaches. However, since general therapeutic and monitoring recommendations can be made regarding all forms of AKI, our approach will be to begin with general measures. Definition and staging of AKI AKI is common, harmful, and potentially treatable. Even a minor acute reduction in kidney function has an adverse prognosis. Early detection and treatment of AKI may improve outcomes. Two similar definitions based on SCr and urine output (RIFLE and AKIN) have been proposed and validated. There is a need for a single definition for practice, research, and public health. 2.1.1: AKI is defined as any of the following (Not Graded): Increase in SCr by ⩾0.3 mg/dl (⩾26.5 μmol/l) within 48 hours; or Increase in SCr to ⩾1.5 times baseline, which is known or presumed to have occurred within the prior 7 days; or Urine volume 4.0 mg/dl (>354 μmol/l), rather than require an acute increase of ⩾0.5 mg/dl (⩾44 μmol/l) over an unspecified time period, we instead require that the patient first achieve the creatinine-based change specified in the definition (either ⩾0.3 mg/dl [⩾26.5 μmol/l] within a 48-hour time window or an increase of ⩾1.5 times baseline). This change brings the definition and staging criteria to greater parity and simplifies the criteria. Recommendation 2.1.2 is based on the RIFLE and AKIN criteria that were developed for average-sized adults. The creatinine change–based definitions include an automatic Stage 3 classification for patients who develop SCr >4.0 mg/dl (>354 μmol/l) (provided that they first satisfy the definition of AKI in Recommendation 2.1.1). This is problematic for smaller pediatric patients, including infants and children with low muscle mass who may not be able to achieve a SCr of 4.0 mg/dl (354 μmol/l). Thus, the pediatric-modified RIFLE AKI criteria 32 were developed using a change in estimated creatinine clearance (eCrCl) based on the Schwartz formula. In pRIFLE, patients automatically reach Stage 3 if they develop an eCrCl 26.5 μmol/l) [within 48 hours or a 50% increase from presumed baseline). Note that a patient can be diagnosed with AKI by fulfilling either criterion 1 or 2 (or 3, urine output) and thus cases B,C,D, and F all fulfill the definition of AKI. Note also that patients may be diagnosed earlier using criterion 1 or 2. Early diagnosis may improve outcome so it is advantageous to diagnose patients as rapidly as possible. For example, case A can be diagnosed with AKI on day 2 by the first criterion, whereas the second criterion is not satisfied until day 3 (increase from 1.3 to 1.9). However, this is only true because the episode of AKI began prior to medical attention, and thus the day 1 SCr level was already increased. If creatinine measurements had available with 48 hours prior to day 1 and if this level had been at baseline (1.0 mg/dl [88.4 μmol/l]), it would have been possible to diagnose AKI on day 1 using the second criterion. Cases F-H do not have a baseline measurement of SCr available. Elevated SCr (reduced eGFR) on day 1 of the hospitalization is consistent with either CKD or AKD without AKI. In Case F, baseline SCr can be inferred to be below the day 1 value because of the subsequent clinical course; thus, we can infer the patient has had an episode of AKI. In case G, AKI can be diagnosed by application of criterion 2, but the patient may have underlying CKD. Case H does not fulfill the definition for AKI based on either criteria, and has either CKD or AKD without AKI. The example of Case A raises several important issues. First, frequent monitoring of SCr in patients at increased risk of AKI will significantly improve diagnostic time and accuracy. If Case A had not presented to medical attention (or if SCr had not been checked) until day 7, the case of AKI would likely have been missed. Frequent measurement of SCr in high-risk patients, or in patients in which AKI is suspected, is therefore encouraged—see Chapter 2.3. The second issue highlighted by Case A is the importance of baseline SCr measurements. Had no baseline been available it would still have been possible to diagnose AKI on day 3 (by either using criterion 2 or by using criterion 1 and accepting the baseline SCr as 1.3); however, not only would this have resulted in a delay in diagnosis, it would have resulted in a delay in staging (see Table 7). On day 7, it can be inferred that the patient's baseline was no higher than 1.0 mg/dl (88 μmol/l) and thus correct staging of Case A as Stage 2 (two-fold increase from the reference SCr, see below and Table 7) on day 3 could have been determined in retrospect. However, if a baseline SCr was available to use as the reference, the correct stage could be determined on day 3. Case B illustrates why criterion 2 can detect cases of AKI missed by criterion 1. It also clarifies why these cases are unusual. Had the SCr increased to 1.5 mg/dl (132.6 μmol/l) as opposed to peaking at 1.4 mg/dl (123.8 μmol/l), it would have been picked up by criterion 1 as well. By contrast Cases C, D, and even F illustrate how criterion 2 may miss cases identified by criterion 1. Note that Case F can only be diagnosed by inference. By day 7, it can be inferred that the baseline was no higher than 1.0 mg/dl (88 μmol/l) and thus it can be determined that the patient presented with AKI. However, if the baseline SCr could be estimated it would be possible to make this inference as early as day 1. Estimating baseline SCr Many patients will present with AKI without a reliable baseline SCr on record. Baseline SCr can be estimated using the Modification of Diet in Renal Disease (MDRD) Study equation assuming that baseline eGFR is 75 ml/min per 1.73 m 2 (Table 9). 22 This approach has been used in many, but not all, studies of AKI epidemiology using RIFLE 2, 5, 25, 30, 31, 32, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63 (see Table 8) and has recently been validated. 64 Hence, most current data concerning AKI defined by RIFLE criteria are based on estimated baseline SCr for a large proportion of patients. Table 9 shows the range of estimated SCr obtained by back-calculation for various age, sex, and race categories. When the baseline SCr is unknown, an estimated SCr can be used provided there is no evidence of CKD (see Appendix B). Fortunately, when there is a history of CKD, a baseline SCr is usually available. Unfortunately, many cases of CKD are not identified, and thus estimating the baseline SCr may risk labeling a patient with AKI when in reality the diagnosis was unidentified CKD. As discussed further in Appendix B, it is essential to evaluate a patient with presumed AKI for presence of CKD. Furthermore, CKD and AKI may coexist. By using all available clinical data (laboratory, imaging, history, and physical exam) it should be possible to arrive at both an accurate diagnosis as well as an accurate estimate of baseline SCr. Importantly, excluding some cases of hemodilution secondary to massive fluid resuscitation (discussed below), the lowest SCr obtained during a hospitalization is usually equal to or greater than the baseline. This SCr should be used to diagnose (and stage) AKI. For example, if no baseline SCr was available in Case A, diagnosis of AKI could be made using the MDRD estimated SCr (Table 9). If Case A were a 70-year-old white female with no evidence or history of CKD, the baseline SCr would be 0.8 mg/dl (71 μmol/l) and a diagnosis of AKI would be possible even on day 1 (criterion 1, ⩾50% increase from baseline). However, if the patient was a 20-year-old black male, his baseline SCr would be estimated at 1.5 mg/dl (133 μmol/l). Since his admission SCr is lower, this is assumed to be the baseline SCr until day 7 when he returns to his true baseline, and this value can be taken as the baseline. These dynamic changes in interpretation are not seen in epidemiologic studies, which are conducted when all the data are present, but are common in clinical medicine. Note that the only way to diagnose AKI (by SCr criteria) in Case H is to use an estimated SCr. Examples of application of AKI stages Once a diagnosis of AKI has been made, the next step is to stage it (Recommendation 2.1.2). Like diagnosis, staging requires reference to a baseline SCr when SCr criteria are used. This baseline becomes the reference SCr for staging purposes. Table 10 shows the maximum stage for each Case described in Table 7. Staging for Case A was already mentioned. The maximum stage is 2 because reference SCr is 1.0 mg/dl (88 μmol/l) and the maximum SCr is 2.0 mg/dl (177 μmol/l). Had the reference SCr been 0.6 mg/dl (53 μmol/l), the maximum stage would have been 3. Case F was staged by using the lowest SCr (1.0 mg/dl [88 μmol/l]) as the reference. Of course, the actual baseline for this case might have been lower but this would not affect the stage, since it is already Stage 3. Note that if this patient was a 35-year-old white male, his MDRD estimated baseline SCr would be 1.2 mg/dl (106 μmol/l) (Table 9) and his initial stage on admission (day 1) would be assumed to be 2. However, once his SCr recovered to 1.0 mg/dl (88 μmol/l) on day 7, it would be possible to restage him as having had Stage 3. Once he has recovered, there may be no difference between Stage 2 or 3 in terms of his care plan. On the other hand, accurately staging the severity of AKI may be important for intensity of follow-up and future risk. Note that Cases G and H can only be staged if the reference SCr can be inferred. Case G may be as mild as stage 1 if the baseline is equal to the nadir SCr on day 7. On the other hand, if this case were a 70-year-old white female with no known evidence or history of CKD, the reference SCr would be 0.8 mg/dl (71 μmol/l) based on an estimated baseline (Table 9). In this case, the severity on day 1 would already be stage 2. Urine output vs. SCr Both urine output and SCr are used as measures of an acute change in GFR. The theoretical advantage of urine output over SCr is the speed of the response. For example, if GFR were to suddenly fall to zero, a rise in SCr would not be detectable for several hours. On the other hand, urine output would be affected immediately. Less is known about the use of urine output for diagnosis and staging compared to SCr, since administrative databases usually do not capture urine output (and frequently it is not even measured, especially outside the ICU). However, studies using both SCr and urine output to diagnose AKI show increased incidence, suggesting that the use of SCr alone may miss many patients. The use of urine output criteria (criterion 3) will also reduce the number of cases where criterion 1 and criterion 2 are discordant (cases B,C,D, and F in Table 7), as many of these cases will be picked up by urine output criteria. Timeframe for diagnosis and staging The purpose of setting a timeframe for diagnosis of AKI is to clarify the meaning of the word “acute”. A disease process that results in a change in SCr over many weeks is not AKI (though it may still be an important clinical entity: see Appendix B). For the purpose of this guideline, AKI is defined in terms of a process that results in a 50% increase in SCr within 1 week or a 0.3 mg/dl (26.5 μmol/l) increase within 48 hours (Recommendation 2.1.1). Importantly, there is no stipulation as to when the 1-week or 48-hour time periods can occur. It is stated unequivocally that it does not need to be the first week or 48 hours of a hospital or ICU stay. Neither does the time window refer to duration of the inciting event. For example, a patient may have a 2-week course of sepsis but only develop AKI in the second week. Importantly, the 1-week or 48-hour timeframe is for diagnosis of AKI, not staging. A patient can be staged over the entire episode of AKI such that, if a patient develops a 50% increase in SCr in 5 days but ultimately has a three-fold increase over 3 weeks, he or she would be diagnosed with AKI and ultimately staged as Stage 3. As with any clinical criteria, the timeframe for AKI is somewhat arbitrary. For example, a disease process that results in a 50% increase in SCr over 2 weeks would not fulfill diagnostic criteria for AKI even if it ultimately resulted in complete loss of kidney function. Similarly, a slow process that resulted in a steady rise in SCr over 2 weeks, and then a sudden increase of 0.3 mg/dl (26.5 μmol/l) in a 48-hour period, would be classified as AKI. Such are the inevitable vagaries of any disease classification. However, one scenario deserves specific mention, and that is the case of the patient with an increased SCr at presentation. As already discussed, the diagnosis of AKI requires a second SCr value for comparison. This SCr could be a second measured SCr obtained within 48 hours, and if it is ⩾0.3 mg/dl (⩾26.5 μmol/l) greater than the first SCr, AKI can be diagnosed. Alternatively, the second SCr can be a baseline value that was obtained previously or estimated from the MDRD equation (see Table 9). However, this poses two dilemmas. First, how far back can a baseline value be retrieved and still expected to be “valid” second, how can we infer acuity when we are seeing the patient for the first time? Both of these problems will require an integrated approach as well as clinical judgment. In general, it is reasonable in patients without CKD to assume that SCr will be stable over several months or even years, so that a SCr obtained 6 months or even 1 year previously would reasonable reflect the patient's premorbid baseline. However, in a patient with CKD and a slow increasing SCr over several months, it may be necessary to extrapolate the baseline SCr based on prior data. In terms of inferring acuity it is most reasonable to determine the course of the disease process thought to be causing the episode of AKI. For example, for a patient with a 5-day history of fever and cough, and chest radiograph showing an infiltrate, it would be reasonable to infer that the clinical condition is acute. If SCr is found to be ⩾50% increased from baseline, this fits the definition of AKI. Conversely, a patient presenting with an increased SCr in the absence of any acute disease or nephrotoxic exposure will require evidence of an acute process before a diagnosis can be made. Evidence that the SCr is changing is helpful in establishing acuity. Clinical judgment While the definitions and classification system discussed in Chapter 2.1 provide a framework for the clinical diagnosis of AKI, they should not be interpreted to replace or to exclude clinical judgment. While the vast majority of cases will fit both AKI diagnostic criteria as well as clinical judgment, AKI is still a clinical diagnosis—not all cases of AKI will fit within the proposed definition and not all cases fitting the definition should be diagnosed as AKI. However, exceptions should be very rare. Pseudo-AKI As with other clinical diagnoses defined by laboratory results (e.g., hyponatremia), the clinician must be cautious to interpret laboratory data in the clinical context. The most obvious example is with laboratory errors or errors in reporting. Erroneous laboratory values should obviously not be used to diagnose disease and suspicious lab results should always be repeated. Another example is when two SCr measurements are obtained by different laboratories. While the coefficient of variation for SCr is very small ( 60, indicating NKD. No indicates GFR <60, and based on prior level of GFR, may indicate stable, new, or worse CKD. Oliguria as a measure of kidney function Although urine flow rate is a poor measure of kidney function, oliguria generally reflects a decreased GFR. If GFR is normal (approximately 125 ml/min, corresponding to approximately 107 ml/kg/h for a 70-kg adult), then reduction in urine volume to <0.5 ml/kg/h would reflect reabsorption of more than 99.5% of glomerular filtrate. Such profound stimulation of tubular reabsorption usually accompanies circulatory disturbances associated with decreased GFR. Oliguria is unusual in the presence of a normal GFR and is usually associated with the non–steady state of solute balance and rising SCr sufficient to achieve the criteria for AKI. As a corollary, if GFR and SCr are normal and stable over an interval of 24 hours, it is generally not necessary to measure urine flow rate in order to assess kidney function. In principle, oliguria (as defined by the criteria for AKI) can occur without a decrease in GFR. For example, low intake of fluid and solute could lead to urine volume of less than 0.5 ml/kg/h for 6 hours or 0.3 ml/kg/h for 24 hours. On the other hand, severe GFR reduction in CKD usually does not lead to oliguria until after the initiation of dialysis. As described in Chapter 2.1, the thresholds for urine flow for the definition of AKI have been derived empirically and are less well substantiated than the thresholds for increase in SCr. Urinary diagnostic indices, such as the urinary concentrations of sodium and creatinine and the fractional reabsorption of sodium and urea, remain helpful to distinguish among causes of AKI, but are not used in the definition (see Appendix D). Kidney damage Table 13 describes measures of kidney damage in AKD and CKD. Kidney damage is most commonly ascertained by urinary markers and imaging studies. Most markers and abnormal images can indicate AKD or CKD, based on the duration of abnormality. One notable exception is small kidneys, either bilateral or unilateral, indicating CKD, which are discussed separately below. Kidney damage is not a criterion for AKI; however, it may be present. Renal tubular epithelial cells and coarse granular casts, often pigmented and described as “muddy brown”, remain helpful in distinguishing the cause of AKI, but are not part of the definition. Small kidneys as a marker of kidney damage Loss of renal cortex is considered a feature of CKD, and is often sought as a specific diagnostic sign of CKD. Kidney size is most often evaluated by ultrasound. In a study of 665 normal volunteers, 69 median renal lengths were 11.2 cm on the left side and 10.9 cm on the right side. Renal size decreased with age, almost entirely because of parenchymal reduction. The lowest 10th percentiles for length of the left and right kidney were approximately 10.5 and 10.0 cm, respectively, at age 30 years, and 9.5 and 9.0 cm, respectively, at age 70 years. Integrated approach to AKI, AKD, and CKD Clinical evaluation is necessary for all patients with alterations in kidney function or structure. The expectation of the Work Group is that the diagnostic approach will usually begin with assessment of GFR and SCr. However, evaluation of kidney function and structure is not complete unless markers of kidney damage—including urinalysis, examination of the urinary sediment, and imaging studies—have been performed. Table 14 shows a summary of the diagnostic approach using measures for kidney function and structure. Based on interpretation of each measure separately, the clinical diagnosis indicated by an “X” can be reached. SPONSORSHIP KDIGO gratefully acknowledges the following sponsors that make our initiatives possible: Abbott, Amgen, Belo Foundation, Coca-Cola Company, Dole Food Company, Genzyme, Hoffmann-LaRoche, JC Penney, NATCO—The Organization for Transplant Professionals, NKF—Board of Directors, Novartis, Robert and Jane Cizik Foundation, Shire, Transwestern Commercial Services, and Wyeth. KDIGO is supported by a consortium of sponsors and no funding is accepted for the development of specific guidelines. DISCLAIMER While every effort is made by the publishers, editorial board, and ISN to see that no inaccurate or misleading data, opinion or statement appears in this Journal, they wish to make it clear that the data and opinions appearing in the articles and advertisements herein are the responsibility of the contributor, copyright holder, or advertiser concerned. Accordingly, the publishers and the ISN, the editorial board and their respective employers, office and agents accept no liability whatsoever for the consequences of any such inaccurate or misleading data, opinion or statement. While every effort is made to ensure that drug doses and other quantities are presented accurately, readers are advised that new methods and techniques involving drug usage, and described within this Journal, should only be followed in conjunction with the drug manufacturer's own published literature.