Review: Neutrophil gelatinase-associated lipocalin: A troponin-like biomarker for human acute kidney injury

Authors


Dr Prasad Devarajan, Cincinnati Children's Hospital Medical Center, University of Cincinnati School of Medicine, MLC 7022, 3333 Burnet Avenue, Cincinnati, OH 45229-3039, USA. Email: prasad.devarajan@cchmc.org

ABSTRACT

Acute kidney injury (AKI) is a common and serious condition, the diagnosis of which currently depends on functional markers such as serum creatinine measurements. Unfortunately, creatinine is a delayed and unreliable indicator of AKI. The lack of early biomarkers of structural kidney injury (akin to troponin in acute myocardial injury) has hampered our ability to translate promising experimental therapies to human AKI. Fortunately, understanding the early stress response of the kidney to acute injuries has revealed a number of potential biomarkers. The discovery, translation and validation of neutrophil gelatinase-associated lipocalin (NGAL), possibly the most promising novel AKI biomarker, is reviewed. NGAL is emerging as an excellent stand-alone troponin-like structural biomarker in the plasma and urine for the early diagnosis of AKI, and for the prediction of clinical outcomes such as dialysis requirement and mortality in several common clinical scenarios. The approach of using NGAL as a trigger to initiate and monitor therapies for AKI, and as a safety biomarker when using potentially nephrotoxic agents, is also promising. In addition, it is hoped that the use of sensitive and specific biomarkers such as NGAL as endpoints in clinical trials will result in a reduction in required sample sizes, and hence the cost incurred. Furthermore, predictive biomarkers like NGAL may play a critical role in expediting the drug development process. However, given the complexity of AKI, additional biomarkers (perhaps a panel of plasma and urinary biomarkers) may eventually need to be developed and validated for optimal progress to occur.

THE NEED FOR A TROPONIN-LIKE BIOMARKER FOR ACUTE KIDNEY INJURY

When a subject presents with symptoms of chest pain, the objective measurement of structural biomarkers such as troponin that are released from damaged myocytes can rapidly identify acute myocardial injury. This has allowed for timely therapeutic interventions, and a dramatic decrease in mortality over the past few decades. An analogous and potentially equally serious condition of the kidney, acute kidney injury (AKI), is largely asymptomatic, and establishing the diagnosis in this increasingly commonly recognized disorder currently hinges on functional biomarkers such as serial serum creatinine measurements. Unfortunately, serum creatinine is a delayed and unreliable indicator of AKI for a variety of reasons.1–3 First, even normal serum creatinine is influenced by several non-renal factors such as age, gender, muscle mass, muscle metabolism, medications, hydration status, nutrition status and tubular secretion. Second, a number of acute and chronic kidney conditions can exist with no increase in serum creatinine due to the concept of renal reserve – it is estimated that greater than 50% of kidney function must be lost before serum creatinine rises. Third, serum creatinine concentrations do not reflect the true decrease in glomerular filtration rate (GFR) in the acute setting, as several hours to days must elapse before a new equilibrium between the presumably steady state production and the decreased excretion of creatinine is established. Fourth, an increase in serum creatinine represents a late indication of a functional change in GFR, which lags behind important structural changes that occur in the kidney during the early damage stage of AKI.4 Indeed, animal studies have identified several interventions that can prevent and/or treat AKI if instituted early in the disease course, well before the serum creatinine even begins to rise. The lack of early biomarkers has hampered our ability to translate these promising therapies to human AKI. Also lacking are reliable methods to assess efficacy of protective or therapeutic interventions, and early predictive biomarkers of drug toxicity.

A troponin-like biomarker of AKI that is easily measured, unaffected by other biological variables, and capable of both early detection and risk stratification would represent a tremendous advance in the care of hospitalized patients, as the incidence of AKI in this population is estimated at a staggering 5–7%.1–3 The incidence of AKI in the intensive care unit (ICU) is even higher – about 25% – and carries an overall mortality rate of 50–80%. In a recent multinational study of AKI in nearly 30 000 critically ill patients, the overall prevalence of AKI requiring renal replacement therapy (RRT) was 5.7% with a mortality rate of 60.3%.5 An increased in morbidity and mortality associated with AKI has been demonstrated in a wide variety of common clinical situations, including those exposed to radiocontrast dye, cardiopulmonary bypass, mechanical ventilation and sepsis.5–7 The negative influence of AKI on overall outcomes in critically ill patients is also well documented.8–10 In addition, recent studies have revealed that AKI is a major risk factor for the development of non-renal complications and it independently contributes to mortality.6 Furthermore, the treatment of AKI represents an enormous financial burden to society. For example, AKI-associated medical expenses have been conservatively estimated at $8 billion per annum in datasets from 23 hospitals in Massachusetts, USA.11

THE CHARACTERISTICS OF AN IDEAL ACUTE KIDNEY INJURY BIOMARKER

Desirable characteristics of clinically applicable AKI biomarkers include: (i) they should be non-invasive and easy to perform at the bedside or in a standard clinical laboratory, using easily accessible samples such as blood or urine; (ii) they should be rapidly and reliably measurable using standardized clinical assay platforms; (iii) they should be sensitive to facilitate early detection, and with a wide dynamic range and cut-off values that allow for risk stratification; and (iv) they should exhibit strong biomarker performance on statistical analysis, including accuracy testing by receiver-operating characteristic curves.12–15

In addition to aiding in the early diagnosis and prediction, they should be highly specific for AKI, and enable the identification of AKI subtypes and aetiologies. AKI is traditionally diagnosed when the kidney's major function (glomerular filtration) is affected, and indirectly measured by change in serum creatinine. However, pre-renal factors such as volume depletion, decreased effective circulating volume or alterations in the calibre of the glomerular afferent arterioles all cause elevations in serum creatinine. Post-renal factors such as urinary tract obstruction similarly result in elevations in serum creatinine. Finally, a multitude of intrinsic renal diseases may result in abrupt rise in serum creatinine, particularly in hospitalized patients. Other tests to distinguish these various forms of AKI such as microscopic urine examination for casts and determination of fractional excretion of sodium have been imprecise and have not enabled efficient clinical trial design. Availability of accurate biomarkers that can distinguish pre-renal and post-renal conditions from true intrinsic AKI would represent a significant advance.

Biomarkers may serve several other purposes in AKI.12–15 Thus, biomarkers are also needed for: (i) identifying the primary location of injury (proximal tubule, distal tubule, interstitium or vasculature); (ii) pinpointing the duration of kidney failure (AKI, chronic kidney disease (CKD) or ‘acute-on-chronic’ kidney injury); (iii) identifying AKI aetiologies (ischaemia, toxins, sepsis or a combination); (iv) risk stratification and prognostication (duration and severity of AKI, need for dialysis, length of hospital stay, mortality); and (v) monitoring the response to AKI interventions. Furthermore, AKI biomarkers may play a critical role in expediting the drug development process. The Critical Path Initiative first issued by the Food and Drug Administration in 2004 stated that ‘Additional biomarkers (quantitative measures of biologic effects that provide informative links between mechanism of action and clinical effectiveness) and additional surrogate markers (quantitative measures that can predict effectiveness) are needed to guide product development’. Collectively, it is envisioned that biomarkers will play an indispensable role in personalizing nephrologic care, by providing a more precise determination of disease predisposition, diagnosis and prognosis, earlier preventive and therapeutic interventions, a more efficient drug development process, and a safer and more fiscally responsive approach to medicine.

Not surprisingly, the pursuit of improved biomarkers for the early diagnosis of AKI and its outcomes is an area of intense contemporary research. For answers, we must turn to the kidney itself. Indeed, understanding the early stress response of the kidney to acute injuries has revealed a number of potential biomarkers.14–17 The bench-to-bedside journey of neutrophil gelatinase-associated lipocalin (NGAL), arguably the most promising novel AKI biomarker, is chronicled in this review.

BIOLOGY OF NGAL

Human NGAL was originally identified as a 25 kDa protein covalently bound to matrix metalloproteinase-9 (MMP-9) from neutrophils.18 Like other lipocalins, NGAL forms a barrel-shaped tertiary structure with a hydrophobic calyx that binds small lipophilic molecules.19 The major ligands for NGAL are siderophores, small iron-binding molecules. On the one hand, siderophores are synthesized by bacteria to acquire iron from the surroundings, and NGAL exerts a bacteriostatic effect by depleting siderophores. On the other hand, siderophores produced by eukaryotes participate in NGAL-mediated iron shuttling that is critical to various cellular responses such as proliferation and differentiation.20 Although NGAL is expressed only at very low levels in several human tissues, it is markedly induced in injured epithelial cells, including the kidney, colon, liver and lung. These findings provide a potential molecular mechanism for the documented role of NGAL in enhancing the epithelial phenotype, both during kidney development and following AKI.18 And finally, NGAL is markedly induced in a number of human cancers, where it often represents a predictor of poor prognosis.21 The over-expressed NGAL protein binds to MMP-9, thereby preventing MMP-9 degradation and increasing MMP-9 enzyme activity. In turn, MMP-9 activity promotes cancer progression by degrading the basement membranes and extracellular matrix, liberating vascular endothelial growth factor, and thus enabling angiogenesis, invasion and metastasis.

NGAL FOR THE PREDICTION OF AKI

Preclinical transcriptome profiling studies identified Ngal (also known as lipocalin 2 or lcn2) to be one of the most upregulated genes in the kidney very early after acute injury in animal models.22,23 Downstream proteomic analyses also revealed NGAL to be one of the most highly induced proteins in the kidney after ischaemic or nephrotoxic AKI in animal models.24–26 The serendipitous finding that NGAL protein was easily detected in the urine soon after AKI in animal studies has initiated a number of translational studies to evaluate NGAL as a non-invasive biomarker in human AKI. In a cross-sectional study of adults with established AKI (doubling of serum creatinine) from varying aetiologies, a marked increase in urine and serum NGAL was documented by western blotting when compared with normal controls.26 Urine and serum NGAL levels correlated with serum creatinine, and kidney biopsies in subjects with AKI showed intense accumulation of immunoreactive NGAL in cortical tubules, confirming NGAL as a sensitive index of established AKI in humans. A number of subsequent studies have now implicated NGAL as an early diagnostic biomarker for AKI in common clinical situations, as shown in Tables 1 and 2 and detailed below.

Table 1.  Urinary neutrophil gelatinase-associated lipocalin for the early prediction of acute kidney injury
ReferenceSettingSubjects (n)SensitivitySpecificityAUC-ROC (CI)
  1. AUC-ROC, area under the receiver-operating characteristic curve; CI, 95% confidence interval; NR, not reported.

27Cardiac surgery711.00.980.99 (NR)
30Cardiac surgery810.730.780.80 (0.57–1.03)
31Cardiac surgery720.490.790.69 (0.57–0.82)
32Cardiac surgery426NRNR0.61 (0.54–0.68)
33Cardiac surgery330.710.730.88 (NR)
67Cardiac surgery1960.820.90.93 (NR)
29Cardiac surgery401.01.01.00 (NR)
34Cardiac surgery500.930.780.96 (0.9–1.0)
46Contrast100NRNRNR
48Contrast400.770.710.73 (0.54–0.93)
49Contrast910.731.00.92 (NR)
59Emergency room6350.90.990.95 (0.88–1.0)
51Critical care1500.770.720.78 (0.62–0.95)
53Critical care310.910.950.98 (0.82–0.98)
54Critical care451NRNR0.71 (0.63–0.78)
43Kidney transplant630.90.830.90 (0.71–1.0)
44Kidney transplant910.770.740.81 (0.70–0.92)
Table 2.  Plasma neutrophil gelatinase-associated lipocalin for the early prediction of acute kidney injury
ReferenceSettingSubjects (n)SensitivitySpecificityAUC-ROC (CI)
  1. AUC-ROC, area under the receiver-operating characteristic curve; CI, 95% confidence interval; NR, not reported.

27Cardiac surgery710.70.940.91 (NR)
31Cardiac surgery72NRNR0.54 (0.4–0.67)
66Cardiac surgery1200.840.940.96 (0.94–0.99)
34Cardiac surgery500.80.670.85 (0.73–0.97)
35Cardiac surgery1000.790.780.8 (0.63–0.96)
50Contrast910.731.00.91 (NR)
52Critical care1430.860.390.68 (0.56–0.79)
55Critical care3010.730.810.78 (0.65–0.90)
57Critical care880.820.970.92 (0.85–0.97)
58Liver transplant590.680.80.79 (NR)

In several prospective studies of children who underwent elective cardiac surgery, AKI (defined as a 50% increase in serum creatinine) occurred 1–3 days after surgery.27–29 In contrast, NGAL measurements by enzyme-linked immunosorbent assay (ELISA) revealed a 10-fold or more increase in the urine and plasma, within 2–6 h of the surgery in those who subsequently developed AKI. Both urine and plasma NGAL were excellent independent predictors of AKI, with an area under the receiver-operating characteristic curve (AUC-ROC) of >0.9 for the 2–6 h urine and plasma NGAL measurements. These findings have now been confirmed in prospective studies of adults who developed AKI after cardiac surgery, in whom urinary and/or plasma NGAL was significantly elevated by 1–3 h after the operation.30–37 However, the AUC-ROC for the prediction of AKI have been rather disappointing when compared with paediatric studies, and have ranged widely from 0.61 to 0.96. The somewhat inferior performance in adult populations may be reflective of confounding variables such as older age groups, pre-existing kidney disease, prolonged bypass times, chronic illness and diabetes.38,39 The predictive performance of NGAL also depends on the definition of AKI employed, as well as on the severity of AKI.37 For example, the predictive value of plasma NGAL post cardiac surgery was higher for more severe AKI (increase in serum creatinine >50%; mean AUC-ROC 0.79) compared with less severe AKI (increase in serum creatinine >25%; mean AUC-ROC 0.65). Similarly, the discriminatory ability of NGAL for AKI increased with increasing severity as classified by Risk, Injury, Failure, Loss, End-stage (RIFLE) criteria. Thus, the AUC-ROC improved progressively for discrimination of R (0.72), I (0.79) and F (0.80) category of AKI.37 Furthermore, the predictive power of urinary NGAL for AKI after cardiac surgery varied with baseline renal function, with optimal discriminatory performance in patients with normal preoperative renal function.40 The variable performance of NGAL after cardiac surgery may also be related to the complex and multifactorial pathogenesis of cardiac surgery-associated AKI. Mechanisms include ischaemia-reperfusion injury (due to low mean arterial pressures and loss of pulsatile renal blood flow), exogenous toxins (due to contrast media, non-steroidal anti-inflammatory drugs, aprotinin), endogenous toxins (due to iron released from haemolysis), and inflammation and oxidative stress (from contact with bypass circuit, surgical trauma and intra-renal inflammatory responses). These mechanisms of injury are likely to be active at different times with different intensities and may act synergistically. Despite these numerous potential variables, a recent meta-analysis of published studies in all patients after cardiac surgery revealed an overall AUC-ROC of 0.78 for prediction of AKI, when NGAL was measured within 6 h of initiation of cardiopulmonary bypass and AKI was defined as a >50% increase in serum creatinine.41 This performance compares favourably with that of troponin for the prediction of myocardial infarction during its clinical implementation period.

Neutrophil gelatinase-associated lipocalin has also been evaluated as a biomarker of AKI in kidney transplantation. In this setting, AKI due to ischaemia-reperfusion injury can result in delayed graft function, most commonly defined as dialysis requirement within the first post-operative week. Protocol biopsies of kidneys obtained 1 h after vascular anastomosis revealed a significant correlation between NGAL staining intensity in the allograft and the subsequent development of delayed graft function.42 In a prospective multicentre study of children and adults, urine NGAL levels in samples collected on the day of transplant identified those who subsequently developed delayed graft function (which typically occurred 2–4 days later), with an AUC-ROC of 0.9.43 This has now been confirmed in a larger multicentre cohort, in which urine NGAL measured within 6 h of kidney transplantation predicted subsequent delayed graft function with an AUC-ROC of 0.81.44 Plasma NGAL measurements have also been correlated with delayed graft function following kidney transplantation from donors after cardiac death.45

Several investigators have examined the role of NGAL as a predictive biomarker of nephrotoxicity following contrast administration.46–50 In a prospective study of children undergoing elective cardiac catheterization with contrast administration, both urine and plasma NGAL predicted contrast-induced nephropathy (defined as a 50% increase in serum creatinine from baseline) within 2 h after contrast administration, with an AUC-ROC of 0.91–0.92.49 In several studies of adults administered contrast, an early rise in both urine (4 h) and plasma (2 h) NGAL were documented, in comparison with a much later increase in plasma cystatin C levels (8–24 h after contrast administration), providing further support for NGAL as an early biomarker of contrast nephropathy.46–48 A recent meta-analysis revealed an overall AUC-ROC of 0.894 for prediction of AKI, when NGAL was measured within 6 h after contrast administration and AKI was defined as a >25% increase in serum creatinine.41

Urine and plasma NGAL measurements also represent early biomarkers of AKI in a very heterogeneous paediatric intensive care setting, being able to predict this complication about 2 days before the rise in serum creatinine, with high sensitivity and AUC-ROC of 0.68–0.78.51,52 Several studies have now examined plasma and urine NGAL levels in critically ill adult populations.53–56 Urine NGAL obtained on admission predicted subsequent AKI in multi-trauma patients with an outstanding AUC-ROC of 0.98.53 However, in a more mixed population of all critical care admissions, the urine NGAL on admission was only moderately predictive of AKI with an AUC-ROC of 0.71.54 In studies of adult intensive care patients, plasma NGAL concentrations on admission constituted a very good to outstanding biomarker for development of AKI within the next 2 days, with AUC-ROC ranges of 0.78–0.92.55,57

In subjects undergoing liver transplantation, a single plasma NGAL level obtained within 2 h of reperfusion was highly predictive of subsequent AKI, with an AUC-ROC of 0.79.58 Finally, in a study of adults in the emergency department setting, a single measurement of urine NGAL at the time of initial presentation predicted AKI with an outstanding AUC-ROC of 0.95, and reliably distinguished pre-renal azotemia from intrinsic AKI and from CKD.59 Thus, NGAL is a useful early AKI marker that predicts development of AKI even in heterogeneous groups of patients with multiple comorbidities and with unknown timing of kidney injury. However, it should be noted that patients with septic AKI display the highest concentrations of both plasma and urine NGAL when compared with those with non-septic AKI,56 a confounding factor that may add to the heterogeneity of the results in the critical care setting. The variable performance of biomarkers such as NGAL in the critical care setting may also be attributable to the fact that this patient population is extremely heterogeneous, and the aetiology and timing of AKI is often unclear. A high proportion of patients may have already sustained AKI on admission to the ICU. Although sepsis accounts for 30–50% of all AKI encountered in critically ill patients, other aetiologies include exposure to nephrotoxins, hypotension, kidney ischaemia, mechanical ventilation and multi-organ disease. Each of these aetiologies is associated with distinct mechanisms of injury that are likely to be active at different times with different intensities and may act synergistically. Despite the myriad confounding variables, a recent meta-analysis revealed an overall AUC-ROC of 0.73 for prediction of AKI, when NGAL was measured within 6 h of clinical contact with critically ill subjects and AKI was defined as a >50% increase in serum creatinine.41

NGAL FOR MONITORING TRIALS IN AKI

Because of its high predictive properties for AKI, NGAL is also emerging as an early biomarker in interventional trials. For example, a reduction in urine NGAL has been employed as an outcome variable in clinical trials demonstrating the improved efficacy of a modern hydroxyethylstarch preparation over albumin or gelatin in maintaining renal function in cardiac surgery patients.60–62 Similarly, the response of urine NGAL was attenuated in adult cardiac surgery patients who experienced a lower incidence of AKI after sodium bicarbonate therapy when compared with sodium chloride.63 In addition, urinary NGAL levels have been used to document the efficacy of a miniaturized cardiopulmonary bypass system in the preservation of kidney function when compared with standard cardiopulmonary bypass.64 Furthermore, adults who developed AKI after aprotinin use during cardiac surgery displayed a dramatic rise in urine NGAL in the immediate post-operative period, attesting to the potential use of NGAL for the prediction of nephrotoxic AKI.65 Not surprisingly, NGAL measurements as an outcome variable are currently included in several ongoing clinical trials formally listed in ClinicalTrails.gov. The approach of using NGAL as a trigger to initiate and monitor novel therapies, and as a safety biomarker when using potentially nephrotoxic agents, is expected to increase. It is also hoped that the use of predictive and sensitive biomarkers such as NGAL as endpoints in clinical trials will result in a reduction in required sample sizes, and hence the cost incurred.

NGAL FOR THE PROGNOSIS OF AKI

A number of studies have demonstrated the utility of early NGAL measurements for predicting the severity and clinical outcomes of AKI. In children undergoing cardiac surgery, early post-operative plasma NGAL levels strongly correlated with duration and severity of AKI, length of hospital stay and mortality.66 In a similar cohort, early urine NGAL levels highly correlated with duration and severity of AKI, length of hospital stay, dialysis requirement and death.67 In a multicentre study of children with diarrhoea-associated haemolytic uraemic syndrome, urine NGAL obtained early during the hospitalization predicted the severity of AKI and dialysis requirement with high sensitivity.68 Early urine NGAL levels were also predictive of duration of AKI (AUC 0.79) in a heterogeneous cohort of critically ill paediatric subjects.51

In adults undergoing cardiopulmonary bypass, those who subsequently required renal replacement therapy (RRT) were found to have the highest urine NGAL values soon after surgery.30–37 Similar results were documented in the adult critical care setting.53–59 Collectively, the published studies revealed an excellent overall AUC-ROC of 0.78 for prediction of subsequent dialysis requirement, when NGAL was measured within 6 h of clinical contact.41 Furthermore, a number of studies conducted in the cardiac surgery and critical care populations have identified early NGAL measurements as a very good mortality marker,30–32,54,55,59 with an overall AUC-ROC of 0.71 in these heterogeneous populations.41 Furthermore, there is now evidence for the utility of subsequent NGAL measurements in critically ill adults with established AKI. Serum NGAL measured at the inception of RRT was an independent predictor of 28-day mortality, with an AUC of 0.74.69

CLINICAL PLATFORMS FOR NGAL MEASUREMENT

With respect to the sample source, the majority of AKI biomarkers described thus far have been measured in the urine. Urinary diagnostics have several advantages, including the non-invasive nature of sample collection, the reduced number of interfering proteins, and the potential for the development of patient self-testing kits. However, several disadvantages also exist, including the lack of sample from patients with severe oliguria, and potential changes in urinary biomarker concentration induced by hydration status and diuretic therapy. Plasma-based diagnostics have revolutionized many facets of medicine, as exemplified by the use of troponins for the early diagnosis of acute myocardial infarction. On the other hand, plasma biomarkers may be confounded by extra-renal sources as well as by subclinical changes in renal elimination. Thus, in the case of AKI, it is important and ideal to develop both urinary and plasma biomarkers.

The majority of NGAL results described in the literature have been obtained using research-based ELISA assays that are currently available from commercial sources such as Bioporto (Gentofte, Denmark) and R&D Systems (Minneapolis, MN, USA). These assays are accurate, but are not practical in the clinical setting. In these regards, a major advance has been the development of a point-of-care kit for the clinical measurement of plasma NGAL (Triage® NGAL Device, Biosite Incorporated, San Diego, CA, USA). In children undergoing cardiac surgery, the increase in plasma NGAL levels measured by the Triage® Device at various time points after cardiopulmonary bypass was proportional to the severity of AKI.66 In terms of diagnostic accuracy, the 2 h plasma NGAL measurement showed an AUC of 0.96, sensitivity of 0.84, and specificity of 0.94 for prediction of AKI using a cut-off value of 150 ng/mL.66 Several addition publications have now confirmed the utility and accuracy of the Triage® NGAL Device in critically ill adults.35–37,55,57 The assay is facile with quantitative results available in 15 min, requires only microlitre quantities of whole blood or plasma, and is currently being tested in multicentre trials for further validation. In addition, a urine NGAL immunoassay has been developed for a standardized clinical platform (ARCHITECT® analyzer, Abbott Diagnostics, Abbott Park, IL, USA). In children undergoing cardiac surgery, the increase in urine NGAL levels determined by ARCHITECT® analyzer at various time points after cardiopulmonary bypass was also proportional to the severity of AKI.67 The 2 h urine NGAL showed an AUC of 0.95, sensitivity of 0.79, and specificity of 0.92 for prediction of AKI using a cut-off value of 150 mg/mL.67 This assay is also easy to perform with no manual pretreatment steps, a first result available within 35 min, and requires only 150 µL of urine. This assay is also currently undergoing multicentre validation in several clinical populations.

BIOLOGIC SOURCES OF URINARY AND PLASMA NGAL

The genesis and sources of plasma and urinary NGAL following AKI require further clarification. Although plasma NGAL is freely filtered by the glomerulus, it is largely reabsorbed in the proximal tubules by efficient megalin-dependent endocytosis.20 Direct evidence for this notion is derived from systemic injection of labelled NGAL, which becomes enriched in the proximal tubule but does not appear in the urine in animals.26 Thus, any urinary excretion of NGAL is likely only when there is concomitant proximal renal tubular injury that precludes NGAL reabsorption and/or increases de novo NGAL synthesis. However, gene expression studies in AKI have demonstrated a rapid and massive upregulation of NGAL mRNA in the distal nephron segments – specifically in the thick ascending limb of Henle's loop and the collecting ducts.20 The resultant synthesis of NGAL protein in the distal nephron and secretion into the urine appears to comprise the major fraction of urinary NGAL. Supporting clinical evidence is provided by the consistent finding of a high fractional excretion of NGAL reported in human AKI studies.20,26 The over-expression of NGAL in the distal tubule and rapid secretion into the lower urinary tract is in accord with its teleological function as an antimicrobial strategy. It is also consistent with the proposed role for NGAL in promoting cell survival and proliferation, given the recent documentation of abundant apoptotic cell death in distal nephron segments in several animal and human models of AKI.70,71

With respect to plasma NGAL, the kidney itself does not appear to be a major source. In animal studies, direct ipsilateral renal vein sampling after unilateral ischaemia indicates that the NGAL synthesized in the kidney is not introduced efficiently into the circulation, but is abundantly present in the ipsilateral ureter.20 However, it is now well known that AKI results in a dramatically increased NGAL mRNA expression in distant organs,72 especially the liver and lungs, and the over-expressed NGAL protein released into the circulation may constitute a distinct systemic pool. Additional contributions to the systemic pool in AKI may derive from the fact that NGAL is an acute phase reactant and may be released from neutrophils, macrophages and other immune cells. Furthermore, any decrease in GFR resulting from AKI would be expected to decrease the renal clearance of NGAL, with subsequent accumulation in the systemic circulation. The relative contribution of these mechanisms to the rise in plasma NGAL after AKI remains to be determined.

LIMITATIONS OF NGAL AS AN AKI BIOMARKER

Clearly, NGAL represents a novel predictive biomarker for AKI and its outcomes. However, NGAL appears to be most sensitive and specific in homogeneous patient populations with temporally predictable forms of AKI. Published studies have also identified age as an effective modifier of NGAL's performance as an AKI biomarker, with better predictive ability in children (overall AUC-ROC 0.93) than in adults (AUC-ROC 0.78). Plasma NGAL measurements may be influenced by a number of coexisting variables such as CKD, chronic hypertension, systemic infections, inflammatory conditions, anaemia, hypoxia and malignancies.19,21,73–75 In the CKD population, NGAL levels correlate with the severity of renal impairment. However, it should be noted that the increase in plasma NGAL in these situations is generally much less than those typically encountered in AKI.

There is an emerging literature suggesting that urine NGAL is also a marker of CKD and its severity.1 In this population, urine NGAL levels are elevated and significantly correlated with serum creatinine, GFR and proteinuria.76–78 Urine NGAL has also been shown to represent an early biomarker for the degree of chronic injury in patients with IgA nephropathy79 and lupus nephritis,80–82 and may be increased in urinary tract infections.83 However, the levels of urine NGAL in these situations are significantly blunted compared with that typically measured in AKI.

In addition, there are a number of limitations pertaining to the biomarker studies published thus far. First, majority of studies reported were from single centres that enrolled small numbers of subjects. Validation of the published results in large multicentre studies will be essential. Second, most studies reported to date did not include patients with CKD. This is problematic, not only because it excludes a large proportion of subjects who frequently develop AKI in clinical practice, but also because CKD in itself can result in increased concentrations of NGAL, thereby representing a confounding variable. Third, many studies reported only statistical associations (odds ratio or relative risk), but did not report sensitivity, specificity and AUC for the diagnosis of AKI, which are essential to determine the accuracy of the biomarker. Fourth, only a few studies with relatively small number of cases have investigated biomarkers for the prediction of AKI severity, morbidity and mortality – results of testing NGAL as a predictor of hard clinical outcomes in large multicentre studies are needed.

Finally, the definition of AKI in the published studies has been based largely on elevations in serum creatinine, which raises the issue of using a flawed outcome variable to analyse the performance of a novel assay. The studies of biomarkers such as NGAL for the diagnosis of AKI may have yielded different results had there been a true ‘gold standard’ for AKI. Instead, using AKI as defined by a change in serum creatinine sets up the biomarker assay for lack of accuracy due to either false positives (true tubular injury but no significant change in serum creatinine) or false negatives (absence of true tubular injury, but elevations in serum creatinine due to pre-renal causes or any of a number of confounding variables that haunt this measurement). It will be crucial in future studies to demonstrate: (i) the association between biomarkers and clinical outcomes such as dialysis, cardiovascular events and death; and (ii) that randomization to a treatment for AKI based on high biomarker levels results in an improvement in kidney function and reduction of adverse clinical outcomes. This should be the next goal for the field.

FUTURE PERSPECTIVES FOR ACUTE KIDNEY INJURY BIOMARKERS

Neutrophil gelatinase-associated lipocalin as an AKI biomarker has successfully passed through the pre-clinical, assay development and initial clinical testing stages of the biomarker development process. It has now entered the prospective screening stage, facilitated by the development of commercial tools for the measurement of NGAL on large populations across different laboratories. But will any single biomarker such as NGAL suffice in AKI? In addition to early diagnosis and prediction, it would be desirable to identify biomarkers capable of discerning AKI subtypes, identifying aetiologies, predicting clinical outcomes, allowing for risk stratification and monitoring the response to interventions. In order to obtain all of this desired information, a panel of validated biomarkers may be needed. Other AKI biomarker candidates may include interleukin-18 (IL-18), kidney injury molecule-1 (KIM-1), cystatin C and liver-type fatty acid binding protein (L-FABP), to name a few.1–3 The availability of a panel of validated AKI biomarkers, such as those illustrated in Figure 1, could further revolutionize and personalize renal and critical care in the near future.

Figure 1.

Temporal profile of urinary biomarkers in patients who develop acute kidney injury after cardiopulmonary bypass (CPB). Data are from the following published references: neutrophil gelatinase-associated lipocalin (NGAL),27 liver-type fatty acid binding protein (L-FABP),29 interleukin-18 (IL-18)28 and kidney injury molecule-1 (KIM-1).84

ACKNOWLEDGEMENTS

Studies cited in this review that were performed by the author's laboratory were supported by grants from the NIH (R01 DK53289, RO1 DK069749 and R21 DK070163). Dr Devarajan is a co-inventor on NGAL patents. Biosite(R) Incorporated has signed an exclusive licensing agreement with Cincinnati Children's Hospital for developing plasma NGAL as a biomarker of acute renal failure. Abbott Diagnostics has signed an exclusive licensing agreement with Cincinnati Children's Hospital for developing urine NGAL as a biomarker of acute renal failure. Dr Devarajan has received honoraria for speaking assignments from Biosite(R) Incorporated and Abbott Diagnostics.

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