Introduction
Acute Renal Failure (ARF) remains a common and significant problem in modern medicine.
The epidemiology and causes of ARF vary according to the clinical setting; in intensive
care units it is estimated that the incidence of ARF ranges from 10%–25%,1,2 with
acute tubular necrosis (ATN) accounting for about 75% of cases.3
Recent data from the BEST kidney investigators has estimated the worldwide prevalence
of ARF that is likely to require renal replacement therapy (RRT) amongst ICU patients
to be as high as 5.7%, and is associated with a high hospital mortality rate.4 Despite
recent major advances in the knowledge of the underlying mechanisms leading to kidney
dysfunction, only small improvements have been made with respect to its prevention
and treatment, and ARF continues to be a major contributor to in-hospital mortality.5,6
While past clinical trials in patients with ARF have been hampered by the lack of
a standardized definition of ARF, recent meetings have resulted in validated stages
of ARF as defined by the RIFLE criteria.7,8 These well-defined stages of ARF allow
current trials to be standardized with respect to measures of kidney dysfunction,
which is essential when evaluating patients for ARF. However, there still remains
much improvements to be made in increasing the sensitivity of current markers of acute
kidney injury (AKI). Creatinine is currently the most widely-used marker of renal
function. Its use in the diagnosis of ARF remains a problem, however, as it often
requires as much as a 50% loss in renal function before creatinine levels rise.7 The
fact that the many different therapeutic agents that have been tried in ARF have shown
very little success is perhaps explained by this delay in diagnosing ARF, which has
been compared to beginning the treatment of acute myocardial infarction 48–72 hours
after the coronary occlusion.9
For these reasons, there has been a search for better markers of early AKI in recent
years, which have seen the advent of several promising new biomarkers of renal function
in this setting. This review will discuss some of these promising biomarkers and their
potential as useful markers of AKI. A literature search performed on MEDLINE/PubMed
using the search terms ‘renal insufficiency, acute’ and ‘biological markers’ yielded
414 results, of which relevant articles were selected from all publication types in
the English language, from either human or animal models.10
Diagnosis of ARF
Current diagnosis of ARF relies on standard markers of renal function, and is usually
accomplished through elevations in the levels of serum creatinine, urea, and urinalysis
changes. Recently, RIFLE criteria have been described with regards to the use of these
indices in the diagnosis of ARF.7 Briefly, ARF was defined as a spectrum initially
starting with a risk of kidney injury (R criterion) with an increase in baseline creatinine
≥50% or urine output ≤0.5 mL/kg/h for 6 hours, to end-stage renal disease (E criterion)
with anuria ≥3 months. Acute tubular necrosis (ATN), being the most common cause of
ARF in hospitalized patients, is sometimes mistakenly used interchangeably with ARF.
ATN, however, is a subset of ARF which results from ischemic or toxic injury to the
renal tubular cells as opposed to other causes of ARF which include prerenal azotemia,
urinary tract obstruction, vasculitis, glomerulonephritis, or interstitial nephritis.
The distinction of ATN from these other causes of ARF has been reviewed in recent
years and remains largely based on a combination of clinical history, physical examination
and the standard laboratory studies mentioned above.7,11,12,13
Plasma creatinine, while being the most commonly used marker of renal function, is
a suboptimal marker of ARF for several reasons. It is well known that factors such
as liver dysfunction (leading to decreased creatinine production from creatine), low
muscle mass, increased tubular secretion of creatinine in low-flow states, drug interference
with tubular handling, and increased volume of distribution in critically ill patients
frequently lead to an overestimation of renal function from serum creatinine measurements.7,11,12,13
Moran and Myers observed various opposed patterns of plasma creatinine change with
regards to the measured glomerular filtration rate (GFR) in the non-steady state that
represents ARF, demonstrating the inaccuracy of simple creatinine measurements in
this setting.14
Blood urea nitrogen (BUN) determination has also been used extensively in the evaluation
of kidney function. Levels of BUN also present several confounding factors as they
depend on exogenous urea load, endogenous production, and tubular reabsorption, which
is increased in states of reduced effective renal perfusion.15 Like creatinine, its
use in the diagnosis of ARF has therefore not been without limitations.
Renal Tubular Cell Proteins and Enzymes
N-acetyl-β-D-Glucosaminidase and tubular brush-border enzymes
Several enzymes originating from proximal tubular cells have been investigated as
markers of necrotic damage whose urinary levels are increased following proximal tubule
injury or dysfunction. Their use as markers of proximal tubular injury has been reviewed
recently.16 The acute or chronic damage of the renal tubular cells is believed to
cause the release of these enzymes from the renal tubules into the urine where they
can be measured. Many urinary enzymes were studied in the last 25 years, originating
broadly from the lysosomes, the brush-border membrane and the cytoplasm of the renal
tubular cells. N-acetyl-β-D-Glucosaminidase (NAG), found predominantly in proximal
tubule lysosomes, has been the most extensively studied of these enzymes in ARF in
various contexts including nephrotoxic agents, ATN, after cardiac surgery or renal
transplantation.17–23 Other enzymes that have been investigated include alkaline phosphatase
(AP), alanine amino-peptidase and γ-glutamyl transpeptidase (GGT), which are enzymes
found in the brush-border membrane, while the different isoforms of glutathione-S-tranferase
(α and γ GST) are cytosolic enzymes.24 The measurement of such enzymes is currently
done by automated assays usually via immunonephelometric methods or ELISA.
However, despite the fact that enzymuria clearly reflects tubular injury, the clinical
significance of enzymuria regarding early diagnosis of ARF has been questionned as
it may not always imply progression towards clinical renal failure.23,25 Likewise,
it has been shown on several occasions that contrast administration during coronary
angiography or arteriography provoked an elevation in urinary NAG, however again without
necessarily progressing towards clinically apparent renal failure.26–28 Similarly,
NAG has been known to be elevated in other types of kidney diseases, such as diabetic
nephropathy, perhaps limiting its usefulness in this setting.29
In a small study involving 26 ICU patients of whom 4 developed ARF (defined as a creatinine
increase ≥50% or ≥0.15 mmol/L) Westhuyzen et al. evaluated urinary NAG, GGT, AP, α/γ
GST, and lactate dehydrogenase (LDH) values at ICU admission in the early detection
of ARF.30 Receiver-operating curve (ROC) analysis for GGT, α and γ-GST, AP and NAG
was performed despite the low number of events (area under curve of 0.95, 0.92, 0.89,
0.86 and 0.84 respectively), while the ROC analysis for LDH and creatinine clearance
was lower albeit still significant. Plasma creatinine detected the onset of ARF between
12–96 hours after admission, and BUN determination was unhelpful in diagnosing ARF.
Tubular enzymuria at admission in the ICU was thus useful in their small ICU sample
in predicting the development of ARF, and more so in ruling out the development of
ARF.
A known limitation of enzymuria, however, concerns the inability of these enzymes
to help differentiate between the different clinical causes of ARF. Chew et al. were
unable to differentiate between the various etiologies of ARF using NAG, intestinal-type
AP and tissue non-specific alkaline phosphatase, in 50 patients with ARF (n = 16 for
prerenal ARF, n = 28 for renal ARF, and n = 6 for post-renal ARF).18 Similarly (see
KIM-1 study below), Han et al. could not differentiate between the causes of ARF in
their cohort of 32 patients with ARF using GGT and AP.31
In an interesting study, Herget-Rosenthal evaluated the usefulness of tubular proteinuria
and enzymuria for predicting the need for renal replacement therapy (RRT) in 73 patients
who developed non-oliguric ARF due to ATN, of whom 26 required RRT.32 All patients
with established non-oliguric ARF fulfilling ATN criteria were included in the study
and had their urine collected on the day of inclusion for levels of α-1 and β-2 microglobulins,
cystatin C, retinol-binding protein, α-GST, GGT, LDH, and NAG. Patients requiring
RRT had significantly increased NAG values as opposed to patients who did not, and
the area under the ROC curve was 0.81 for urinary NAG values above 4.5 U/mmol of urinary
creatinine. However the sensitivity/specificity of urinary NAG for RRT was lower than
that of cystatin C and α-1-microglobulin, being 85%/62%. Patients who required RRT
also had significantly higher urinary values of cystatin C and α-1-microglobulin than
patients who did not require RRT. The other markers evaluated (retinol-binding protein,
β-2-microglobulin, GST, GGT, LDH) showed less ability to predict the need for RRT.
Liangos et al. obtained concordant results lately in a study that evaluated both NAG
and KIM-1 (see below for KIM-1). Higher levels of both NAG and KIM-1 were associated
with higher odds ratio for dialysis requirement or hospital death in a larger cohort
of 201 patients separated in four different quartiles according to NAG values.33 Of
note, there were no statistical differences in creatinine values between the 4 different
quartiles, despite differences in the proportion of patients requiring RRT. Unfortunately,
the NAG values for the four different quartiles were not reported.
Overall, these studies indicate that quantifying enzymuria remains a sensitive, albeit
not very specific, tool for establishing the presence of a renal tubular injury. Recent
studies have suggested a promising role for NAG in helping to predict adverse outcomes
in ARF, and future studies should try to validate these results in larger cohorts.
Kidney injury molecule-1 (KIM-1)
KIM-1 was originally described as a putative epithelial cell adhesion molecule upregulated
in rat renal proximal tubules after renal ischemic injury by Ichimura et al. in 1998.34
While its role remains unclear, it consists in a transmembrane protein whose ectodomain
is shed into the urine where it can be measured. In a subsequent study involving 32
patients with various forms of acute and chronic renal disease published in 2002,
urinary KIM-1 levels were shown to be significantly more elevated in patients with
ATN as opposed to patients with other causes of ARF or chronic renal disease.31 The
7 patients with ischemic ATN had mean normalized urinary KIM-1 values of 2.92 +/−
0.61, while the remaining patients with ARF had mean values of 0.63 +/− 0.17, p ≤
0.01 (16 total patients, 7 with contrast nephropathy, 5 with prerenal azotemia, 2
with cyclosporine toxicity, 1 with post-obstructive nephropathy and 1 with interstitial
nephritis). Patients with chronic renal diseases had mean normalized values of 0.72
+/− 0.37, p ≤ 0.01 (n = 9, diseases included Wegener’s granulomatosis, systemic lupus
erythematosus nephropathy, diabetic nephropathy, focal segmental glomerulosclerosis,
and chronic allograft dysfunction). Of note, the definition of ARF used in the study
was 1) increase in serum creatinine ≥0.5 mg/dL if baseline ≤2.0 mg/dL, or 2) increase
in serum creatinine ≥1.5 mg/dL if baseline ≥2.0 mg/dL, or 3) increase in serum creatinine
≥0.5 mg/dL regardless of baseline if as a consequence of exposure to contrast agents.
Urinary AP and GGT were also measured but were not helpful in differentiating ATN
from other causes of renal injury.
Since then, urinary KIM-1 levels have been shown to be increased in rodents receiving
nephrotoxic doses of cisplatin, folic acid, cadmium, gentamycin, mercury and chromium,35–39
suggesting a role of KIM-1 for detecting nephrotoxic insults. Also, Liangos et al.
demonstrated very recently in a cohort of 201 patients with ARF separated in quartiles
according to their urinary levels of KIM-1, that higher levels correlated with a higher
odds ratio for dialysis requirement or hospital death, adding some prognostic value
to the measured levels of urinary KIM-1.33 As with NAG values however, the actual
values of the different KIM-1 by quartiles were not reported. These results all suggest
a useful role of KIM-1 in detecting renal ischemic injuries, but further studies from
larger cohorts will be required to validate this biomarker in current practice.
Na+/H+ exchanger isoform 3 (NHE3)
The NHE3 is the most abundant of all recently detected sodium transporters expressed
in the nephron and can be detected in urine.40 It is localized in the apical membrane
and subapical endosomes of renal proximal tubular cells and in the apical membrane
of thick ascending limb cells,41–43 and is responsible for 60%–70% of the reabsorption
of the filtered sodium.44 Du Cheyron et al. measured the urinary levels of NHE3 in
68 patients admitted to the ICU, 54 of whom either had or developed ARF during their
ICU stay (defined as an increase in serum creatinine to ≥177 μmol/L or to a value
≥50% of basal creatinine when chronic renal insufficiency existed).45 Patients with
ARF were divided into 3 subgroups according to clinical criteria: prerenal azotemia
(which improved rapidly after volume replacement), ATN (defined as renal dysfunction
that did not improve after correction of possible prerenal causes and could not be
attributed to other causes), and intrinsic ARF other than ATN. In this population,
levels of urinary NHE3 normalized for urine creatinine were increased six times as
much in patients with ATN than in those with prerenal azotemia (0.78 +/− 0.36 vs.
0.12 +/− 0.08, p ≤ 0.001), while urinary NHE3 could not be detected in the 14 control
ICU patients without ARF. Urinary RBP was also measured but could not discriminate
between prerenal ARF and ATN.
Urinary actin
Kwon et al. evaluated urinary actin in a study that also looked at interleukin-6 (IL-6),
interleukin-8 (IL-8), GGT, LDH, and tumor necrosis factor-alpha (TNFα) in 40 kidney
transplant recipients during the first post-transplant week.46 Actin is the main cytoskeletal
protein in cells, and damage to the cellular cytoskeleton has been described as one
of the key events following renal ischemia-reperfusion injury.47,48 Of these 40 patients,
9 out of the 30 patients receiving cadaveric allografts progressed to sustained ARF
(defined as a creatinine clearance ≤25 mL/min by 24 hour urine collection at day 7
post-operatively), while the remaining 21 went on to a recovery phase (defined as
a creatinine clearance above 25 mL/min). All 10 patients receiving living donor allografts
progressed to a recovery phase. In their study urinary actin at day 0 was elevated
in recipients destined to have sustained ARF, with ROC analyses for urinary actin
at day 0 yielded values of 0.75. This suggests that urinary actin might serve as a
predictor of sustained ARF after kidney transplantation from cadaveric allografts,
but larger studies will be required to further explore the role of urinary actin in
AKI.
Exosomal fetuin A
All nephron segments secrete exosomes containing apical membrane and intracellular
fluid into the urine under normal conditions and thus may potentially carry protein
markers of structural damage to the nephrons. Exosomal fetuin A, an acute phase protein
involved in various inflammatory states, was very recently identified using urinary
proteomics techniques from a rat models of cisplatin-induced nephrotoxicity and ischemic-reperfusion
injury as a potential marker of AKI in these settings.49 Urinary exosomal fetuin A
was then also shown in the same study to be elevated in 3 ICU patients with AKI compared
to the patients without AKI. Fetuin A might thus be a potential urinary biomarker
of AKI, and will need to be investigated in larger studies.
Cysteine-rich protein 61 (Cyr61)
Cyr61 is a secreted heparin-binding protein that may possibly be involved in tissue
growth and repair,50,51 which was originally described as a growth factor-inducible
immediate early gene in fibroblasts.52 It was identified by Muramatsu et al. in 2002
by representational difference analysis as one of the genes that was rapidly induced
following ischemia in rodent models of ischemic/reperfusion injury.53 The urinary
levels of Cyr61 in rodents could be detected as early as three to six hours following
renal injury, where it might eventually serve as an early biomarker of ARF. More studies
will be required to validate its use in humans.
Sulfated HNK-1 epitope
The HNK-1 carbohydrate epitope is attached to lactosamine structures on glycoproteins,
proteoglycans, or glycolipids and has been known to be an important actor in neurogenesis
and immune systems54,55 through proposed involvement in cell-cell interactions and
cell migration.56 Such interactions are likely to be involved in renal morphogenesis,
as Allory et al. demonstrated in 2006 the expression of the HNK-1 epitope mainly to
the thin ascending loop of Henle in adult kidneys.57 Furthermore, in 10 kidney biopsies
from patients with ATN after renal transplantation, the HNK-1 epitope was found to
be expressed at various levels in 8 of these cases, thus raising the possibility of
its use as a potential marker for ATN.
Urinary Low-Molecular Weight Proteins
Urinary excretion of low-molecular weight (LMW) proteins, like that of renal proximal
tubular enzymes, has also been extensively described in the biomarker literature over
the last 25 years. These LMW proteins are usually freely filtered and then reabsorbed
by the proximal tubule under normal conditions.58 In settings of tubular damage where
such reabsorption is impaired, or in contexts of increased reabsorptive load with
increased transglomerular passage of proteins, these proteins cannot be entirely reabsorbed
and are therefore secreted in urine.59 These proteins that have been used as biomarkers
of tubular dysfunction include β-2-microglobulin, α-1-microglobulin, retinol-binding
protein (RBP), and cystatin C.
In a study from 2004 described previously, Herget-Rosenthal evaluated the usefulness
of tubular proteinuria and enzymuria for predicting the need for RRT in 73 patients
who developed non-oliguric ARF due to ATN, of whom 26 required RRT.32 Patients who
required RRT had significantly higher urinary values of cystatin C and α-1-microglobulin
than patients who did not require RRT. The area under the ROC curves for these two
proteins was 0.92 and 0.86 respectively, and with a sensitivity/specificity of 92%/83%
and 88%/81% respectively, at values of urinary cystatin C ≥1g/mol of creatinine and
α-1-microglobulin ≥20g/mol of creatinine. As said previously, patients requiring RRT
also had significantly increased NAG values as opposed to patients who did not, however
the sensitivity/specificity of urinary NAG for RRT was lower than that of cystatin
C and α-1-microglobulin. The other markers evaluated (RBP, β-2-microglobulin, GST,
GGT, LDH) showed less ability to predict the need for RRT.
While these results appear promising for cystatin C and α-1-microglobulin, previous
studies involving the latter had shown that while being a sensitive marker of tubular
injury, elevations in α-1-microglobulin were not always associated with clinically
relevant renal injury,60–62 which is similar to the use of several renal biomarkers
described previously in this review.
Cystatin C
Cystatin C is a low molecular weight protein functioning as a lysosomal cysteine protease
inhibitor, and is therefore strongly implicated in the regulation of proteolytic damage
of these enzymes.63 Cystatin C is considered one of the housekeeping genes and is
produced at a constant rate by all nucleated cells, without changes with aging, gender,
or muscular mass. By virtue of its low molecular weight, it is freely filtered at
the glomerulus and is completely catabolized by proximal tubules.64–66 These characteristics
thus make cystatin C a good marker of glomerular filtration rate whose reliability
is comparable or superior to plasma creatinine, and has been demonstrated in several
clinical settings.65,67–72 In ARF, however, there exist only a few clinical studies
which evaluated cystatin C. Hergert-Rosenthal demonstrated the clinical significance
of cystatin C in ARF.73 In this study, serum cystatin C and serum creatinine were
measured daily in 85 critically ill patients at high risk of developing ARF. In the
44 patients with ARF (classified according to the recent RIFLE criteria7), a significant
early rise in cystatin C detected a risk of renal injury (R criteria, elevation of
≥50% from baseline creatinine) 1.5 +/− 0.6 days before a rise in serum creatinine.
Applying the same analysis to renal injury and renal insufficiency (I and F criteria)
also detected these conditions earlier in a similar fashion. Regarding the need for
RRT, an elevation of cystatin C ≥50% predicted RRT requirements moderately well, with
a sensitivity of 53% and specificity of 82%. Herget-Rosenthal also demonstrated the
use of urinary cystatin C in 2004 in predicting the need for RRT in patients with
non-oliguric ARF due to ATN, as just described.32
Also, Ahlström et al. measured serum cystatin C daily starting from admission to the
ICU in 202 patients.74 ARF (defined here as a threefold increase in baseline creatinine,
or in case of chronic renal failure as an increase ≥0.5 to 4.0 mg/dL of creatinine,
or diuresis ≤0.3 mL/kg/h for 24 h, or the need for RRT) occurred in 54 patients, and
cystatin C was as useful as creatinine in detecting ARF. Plasma creatinine, however,
was already at a mean of 255 μmol/L at ICU admission in the group of patients that
developed ARF (with mean peak values of plasma creatinine of 294 μmol/L), and RRT
was started on day 2 on average, suggesting pre-existing advanced renal dysfunction
at the time of study inclusion. When only considering abnormal values of creatinine
that developed after ICU admission (defined in the protocol as ≥90 or 95 μmol/L for
females and males, respectively), both serum cystatin C and plasma creatinine appeared
equally quickly (median 3 days). It is unspecified if any of those 29 patients went
on to develop ARF, however.
Of note, one small study has produced conflicting results regarding the use of cystatin
C in ARF. It involved 29 critically ill septic patients of which 10 developed ARF
(defined here as creatinine ≥267 μmol/L or diuresis ≤30 mL/h, duration not specified),
cystatin C was not found to be correlated with the occurrence of ARF, as opposed to
measurements of the N-terminal prohormone of atrial natriuretic peptide (proANP).75
It is unclear from this study how many patients required RRT.
Still, another small study from Delanaye et al. showed that the ability of cystatin
C to detect a GFR ≤ 80 cc/min/1.73m2 (GFR calculated by Cockroft-Gault estimation
or by 24h creatinine clearance) was superior than plasma creatinine in 14 ICU patients.76
Similarly, a report from ten patients undergoing unilateral nephrectomy for transplantation
purposes also demonstrated that serum cystatin C increased by 50 to 100% postoperatively
1.4 +/− 0.9 days earlier than creatinine (p = 0.009).77
Finally, in 2006, a study by Zhu et al. demonstrated the ability of serum cystatin
C in detecting acute renal dysfunction (defined as a creatinine clearance below 80
mL/min/1.73m2 measured by 24 h urine collection) in 60 patients undergoing heart valve
replacement.78 Cystatin C levels peaked at post-operative day 2 on average as opposed
to day 3 for creatinine, and cystatin C levels significantly rose in 19 of the 26
patients developing acute renal dysfunction while only 7 of these patients demonstrated
an elevated serum creatinine. Interestingly, ‘low-dose’ corticosteroid therapy (dexamethasone
10 mg daily for 3 days after surgery, n = 26) did not result in any changes in measured
serum cystatin C when compared to patients not receiving any corticosteroids. Increased
serum cystatin C levels have been reported in patients receiving corticosteroids,79–82
but other investigators have reported so only after large, prolonged doses.80
It should also be noted that in settings of radio-contrast nephropathies, an evaluation
of the nephrotoxic effect of contrast administered during coronary angiography demonstrated
a significant elevation of cystatin C (+7.2%) 24 hours after angiography, while the
serum creatinine rise could only be seen 48 hours after, also suggesting a role of
cystatin C in the diagnosis of AKI.83 Likewise, in a study by Bachorzewska-Gajewska
evaluating NGAL in coronary angiographies, the levels of serum cystatin C were also
significantly elevated 24 hours after coronary angiography (from 1.69 +/−1.03 to 2.85
+/−2.05 mg/L, p < 0.01), while serum creatinine remained unaffected.84
Cystatin C is a well-established marker of GFR in settings of stable renal function
and chronic renal insufficiency.65–72 Taken together, the results of the studies just
described also seem to suggest a possible role for serum cystatin C in detecting AKI,
and perhaps especially for urinary cystatin C, also in predicting adverse outcomes
in ARF such as RRT. In view of the several conflicting studies, it seems likely however
that larger studies will be required to clarify the role of cystatin C in ARF.
Neutrophil gelatinase-associated lipocalin (NGAL)
NGAL is a part of the lipocalin protein family and is a 23 kDa low molecular weight
protein secreted by various types of human cells, which include not only activated
neutrophils85 but also other tissues such as the kidneys, and cells of the gastro-intestinal
and respiratory tracts.86–91 NGAL is also strongly expressed by various types of carcinomas
and adenomas. Its physiologic role seems complex, implying cell growth and differentiation,
a bacteriostatic immune effect and a role in cellular iron-transport pathways.92 By
virtue of its small size NGAL is freely filtered by the renal glomeruli without being
reabsorbed, and can therefore be measured in the urine. Early studies by Mishra et
al. in 2003 using cDNA microarray assay methods allowed its identification as a protein
strongly expressed in renal tubules in animal models of renal ischemic injury.93 Interestingly,
NGAL might act in a protective fashion for the renal tubules in this context, as seem
to indicate some animal studies.94
In humans, the role of this new biomarker in settings of AKI has now been demonstrated.
In a recent study by Mishra et al. urinary and plasma NGAL levels were measured in
71 children undergoing cardiac surgery necessitating extra-corporeal cardio-pulmonary
bypass.95 Amongst the 20 children developing AKI (defined as an elevation of serum
creatinine >50% from baseline), a significant rise in the average urinary NGAL levels
was seen as early as 2 hours post-operatively (from 1.6 μg/L SE 0.3, to 147 μg/L SE
not available), while a significant change in serum creatinine could only be noted
from 24 to 72 hours postoperatively. Using a urinary NGAL cutoff value of 50 μg/L
and looking at the 51 children not developing AKI as case-controls, the urinary NGAL
level 2 hours after CPB showed an impressive 100% sensitivity and 98% specificity
in the early diagnosis of AKI. Plasma NGAL levels showed similar results although
sensitivity and specificity of the test were somewhat lower than for urinary NGAL
in the study. A similar study in adults after cardiac surgery also demonstrated a
significant increase in urinary NGAL 1 hour after surgery, however without being adequately
powered to obtain sensitivity and specificity values.96
Interestingly, Bachorzewska-Gajewska recently demonstrated a rise in urinary NGAL
4 hours following the administration of contrast during percutaneous coronary angioplasties
in 25 adult patients (from 11.1 μg/L +/−15.8 to 17.8 +/− 34.48, p < 0.05) despite
however any significant change in average serum creatinine values.84 These studies
thus suggest a very promising role for NGAL in the early evaluation of ARF which will
require further investigations.
Urinary interleukin-18 (IL-18) and other cytokines
IL-18 is a cytokine with several important roles in human immune defense mechanisms,
principally acting as a co-stimulator in the production of gamma-interferon, a crucial
element in the human defenses against infections. At the renal level, recent animal
studies in mice by Melnikov demonstrated the possible role played by IL-18 in the
renal tubules in settings of acute ischemic tubular necrosis.97,98
It was then demonstrated in humans that the urinary concentration of IL-18 was higher
in ATN when compared with other causes of renal insufficiency such as hypovolemia,
heart failure, urinary tract infections and chronic renal failure (n = 72).99 In the
same study, the urinary IL-18 level at 24 hours after kidney graft was also much higher
in patients who later developed delayed graft dysfunction. The same investigators
also looked at a cohort of 72 children undergoing cardiac surgery requiring cardio-pulmonary
bypass.100 They demonstrated a significant increase in the urinary IL-18 level starting
only 4 hours after surgery in the 20 children who developed an acute renal injury
(as defined by an increase >50% of baseline creatinine in the 3 days after surgery)
when compared to 35 case-controls chosen amongst the 50 children without renal insufficiency.
The sensitivity and specificity of IL-18 in this context seemed best between 12 and
24 hours with values of 40%–50% and 94% respectively, for urinary IL-18 levels higher
than 50 pg/mL. The area under the ROC was 0.73 and 0.75 for these levels of IL-18
at 12 and 24 hours. One other case-control study with urinary samples from 52 cases
and 86 control-cases (controlled for demographic factors, sepsis, baseline creatinine,
APACHE III score, and diuresis) involved in the ARDS Network Study demonstrated a
relationship between a rise >100 ng/mL of IL-18 at 24 and 48 hours and the development
of ARF.101 This rise in urinary IL-18 was also associated with higher mortality amongst
these patients.
These studies thus indicate a possible role for urinary IL-18 in the early evaluation
of ARF, and also an ability to differentiate between some of the causes of ARF. Once
again, more studies will be needed to validate these early results.
Interleukin-6, interleukin-8, and tumor necrosis factor-alpha are other inflammatory
cytokines that have been implicated in the cascade of cellular events that follows
renal ischemic injury.102–104 Their role in the detection of AKI was investigated
in a study by Kwon et al. (mentioned previously) in the context of early kidney transplant
graft rejection.46 In their study, IL-6 and IL-8 (but not TNFα) at day 0 were elevated
in recipients destined to have sustained ARF. ROC analyses for urinary IL-6, and IL-8
at day 0 yielded values of 0.91, and 0.82 respectively. Such results thus suggest
that these markers might serve as strong predictors of sustained ARF after kidney
transplantation, but larger studies should obviously be done to ascertain these early
results.
The Future Search for Biomarkers
As several recent reviews have mentioned, the recent advent of several new biomarkers,
each representing a different aspect of the various causes of ARF (see Table 1), is
likely to continue as our understanding of the different molecular mechanisms involved
in ARF evolves.105,106 Through genomics and proteomics, recent studies have looked
at the different genes and proteins expressed during different models of ARF and identified
several other candidate biomarkers.107 Several studies using cDNA microarray methods
have already identified many genes that are either upregulated or downregulated in
animal models of ARF.108,113–115 A recent study also demonstrated the differential
gene expression between different models of ARF that included nephrotoxic injury,
ischemic injury and hypovolemia.108 As seen in this study however, a frequent problem
with genomic approaches is that modifications in gene transcription do not always
result in altered protein expression because of various post-transcriptional events.
Changes at the genomic level therefore also need to be evaluated in terms of changes
in functional protein levels. Another exciting approach has been through the recent
development of the field of proteomics, and particularly of the urinary proteome.109
This approach focuses on the pattern of urinary protein expression using mass spectrometry
to identify the proteins implicated with the cause of renal disease. In recent years,
proteomics has proven to be a powerful tool in investigation and clinical medicine,
and will likely eventually provide us with a comprehensive knowledge of the various
proteins that can be found in the urine under many conditions.109–112 An important
problem with proteomics is however to ensure the stability of the proteome from the
collection to analysis for there are several factors that might alter samples.112
Also the functional role of the various proteins identified might not necessarily
be directly proportional to their amount, and the identification of scarce compounds
requires methods with high sensitivity which can prove to be difficult. Finally, standards
will have to be established for means of comparison. Still, the research potential
of these methods is obviously enormous, and one should expect large amounts of data
to come forward from these in the next years as our knowledge and techniques evolve.
There are therefore many different biomarkers of kidney function that each reflect
different aspects of renal physiology, and which are all affected differently in the
various conditions that result in ARF. Genomics and proteomics have already yielded
extremely interesting insights in our understanding of the pathophysiology of the
various forms of ARF and their associated potential biomarkers, and will undoubtebly
continue to do so in the next few years. It is likely that no single marker will prove
to have the required sensitivity and specificity required for all these different
diagnoses, and we will most likely depend on a panel of some of the markers implicated
in the various forms of ARF in its different phases to properly diagnose ARF in the
future.