http://informahealthcare.com/rnf ISSN: 0886-022X (print), 1525-6049 (electronic) Ren Fail, 2014; 36(6): 979–986 ! 2014 Informa Healthcare USA, Inc. DOI: 10.3109/0886022X.2014.900429

STATE OF THE ART REVIEW

Neutrophil gelatinase-associated lipocalin (NGAL): a promising biomarker of contrast-induced nephropathy after computed tomography Vassilis Filiopoulos, Dimitra Biblaki, and Dimosthenis Vlassopoulos

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Department of Nephrology, Sismanogleion-Amalia Fleming General Hospital, Athens, Greece

Abstract

Keywords

Contrast-induced nephropathy (CIN) is a common cause of hospital-acquired acute kidney injury (AKI) and a source of significantly increased short- and long-term mortality. Studies of large cohorts have revealed that more than half of these cases are in subjects undergoing cardiac catheterization and intra-arterial coronary angiography, and nearly a third follow computed tomography (CT) scans. Neutrophil gelatinase-associated lipocalin (NGAL) represents an early predictive troponin-like biomarker for AKI. Its role in the timely diagnosis of CIN has already been examined in adults and children undergoing coronary angiography and a metaanalysis revealed a very good performance of plasma or urine NGAL in the prediction of CIN. Much of these data have been extrapolated to patients receiving intravenous (IV) contrast agent for CT scans, although major differences in patient populations, contrast volume administered and intra-procedural complications between the two settings exist. In this context, a recent prospective study by our group evaluated plasma NGAL, measured using standardized SriageÕ NGAL test (Biosite Incorporated, San Diego, CA) at baseline and 6-h postprocedure, for early detection of CIN among hospitalized patients undergoing elective contrastenhanced CT. CIN, defined as an increase in serum creatinine (SCr) of 425% or 40.5 mg/dL from baseline within 48-h post-procedure, was found in 8.51% of subjects. In contrast, significant elevation of plasma NGAL was found at 6-h post-procedure with excellent performance characteristics. This review presents the current status of NGAL in the prediction of CIN after IV contrast administration among hospitalized patients undergoing elective contrast-enhanced CT.

Acute kidney injury, biomarker, computed tomography, contrast-induced nephropathy, neutrophil gelatinase-associated lipocalin

Introduction Contrast-induced nephropathy (CIN) is a common cause of hospital-acquired acute kidney injury (AKI), accounting for up to 12% of cases, and is associated with an average in-hospital mortality of 6%.1 The exact risk for CIN is difficult to determine. The reported incidence of CIN varies widely depending on the presence or absence of risk factors, in particular pre-existing chronic kidney disease (CKD), the amount and type of administered contrast agent and the nature of the radiologic procedure.1–3 However, despite the widespread use of newer, less nephrotoxic contrast agents and the implementation of preventive strategies over the last decades, the risk of CIN continues to be considerable, especially in the in-patient setting.3 Studies of large cohorts have revealed that more than half of CIN cases complicate cardiac catheterization and intra-arterial coronary angiography, and nearly a third follow computed tomography (CT) scans.2 Advances in diagnostic and interventional imaging techniques have Address correspondence to Vassilis Filiopoulos, Department of Nephrology, Sismanogleion-Amalia Fleming General Hospital, 25th Martiou 14, 15127, Athens, Greece. Tel: +30 210 8038391; Fax: +30 213 2003436; E-mail: [email protected]

History Received 11 December 2013 Revised 5 February 2014 Accepted 21 February 2014 Published online 27 March 2014

contributed to a continuously increasing number of patients exposed to iodinated contrast media. Indeed, the number of contrast-enhanced CT scans has been increasing by approximately 2 million per year in the United States over the last decade.3 CIN is an acute decline in renal function that occurs 48–72 h after intravascular injection of contrast medium. The most commonly used clinical definition is a relative rise in serum creatinine (SCr) of at least 25% from the baseline value or an absolute increase in SCr by at least 0.5 mg/dL (44 mmol/ L) within 48–72 h after contrast administration, in the absence of other obvious causes.1–3 However, other definitions are also in use. Indeed, the Acute Kidney Injury Network proposed using the same standardized definition in all cases of AKI: increase in SCr of at least 0.3 mg/dL or at least 50% from baseline or reduction in urine output (50.5 mL/kg/h for 46 h) within 48 h after contrast exposure.3 This definition, however, has not been widely adopted as yet. Furthermore, the criterion of oliguria does not apply for most cases of CIN. AKI due to CIN is largely asymptomatic and establishing the diagnosis is currently based on functional biomarkers such as serial SCr measurements. Unfortunately, SCr is a delayed and not always a reliable indicator of AKI.4 Over 50% of

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Table 1. Principal biomarkers for early detection of AKI in humans.

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Acronym/Abbreviation AP a1MG b2MG CarbHb CysC FENa gGT GST HGF IL-6 IL-8 IL-10 IL-18 KIM-1 LDH L-FABP MMP-9 NGAL NAG Neutrophil CD11b NHE3 RBP Pro-ANP TNF-a IGFBP7 TIMP-2

Legend Alkaline phosphatase Alpha 1 microglobulin Beta 2 microglobulin Carbamylated hemoglobin Cystatin C Fractional excretion of sodium Gamma glutamyl transpeptidase Glutathione S transferase Hepatocyte growth factor Interleukin 6 Interleukin 8 Interleukin 10 Interleukin 18 Kidney injury molecule 1 Lactate dehydrogenase Liver-type fatty acid-binding protein Matrix metalloproteinase-9 Neutrophil gelatinase-associated lipocalin N-acetyl beta glucosaminidase Neutrophil CD11b Sodium hydrogen exchanger 3 Retinol binding protein Prohormone of atrial natriuretic peptide Tumor necrosis factor alpha Insulin-like growth factor-binding protein 7 Tissue inhibitor of metalloproteinases-2

kidney function must be lost before SCr begins to rise. Animal studies have identified several interventions that can prevent and/or treat AKI if instituted early in the disease course.5 The lack of early biomarkers has hampered our ability to translate these promising therapies to human AKI. Therefore, the pursuit of biomarkers with improved performance in early and accurate diagnosis is an area of intense contemporary research with the ultimate goal to implement currently available therapeutic interventions in a timely manner and, consequently, reduce the unacceptably high morbidity and mortality rates associated with AKI.6 Over the last decade, the application of innovative technologies has identified, in urine and plasma or serum, several candidates that are emerging as biomarkers for the early detection of AKI (Table 1).4,6,7 Neutrophil gelatinaseassociated lipocalin (NGAL) is the most promising novel AKI biomarker.7 The role of NGAL in the early diagnosis of CIN has already been established in adults and children undergoing coronary angiography, whereas its performance in patients undergoing contrast-enhanced CT is unclear and most of the data come from extrapolation. The role of NGAL as an AKI biomarker and the current status of NGAL in the prediction of AKI due to CIN and, in particular, after IV contrast administration for CT scan are reviewed in this article. NGAL biology NGAL is also known as lipocalin-2 (LCN2) and was originally identified as a 25-kDa protein covalently bound to gelatinase from secondary granules of human neutrophils.8 Mature peripheral neutrophils lack NGAL mRNA expression, and NGAL protein is synthesized at the early-myelocyte stage of granulopoiesis during the formation of secondary granules. NGAL mRNA is normally expressed in a variety of adult human tissues, including bone marrow, uterus, prostate,

Figure 1. NGAL structure. Note: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no FrontCover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License. (From Wikipedia, the free encyclopedia.)

salivary gland, stomach, colon, trachea, lung, liver and kidney.9 Several of these tissues are prone to exposure to micro-organisms, and constitutively express the NGAL protein at low levels. The promoter region of the NGAL gene contains binding sites for a number of transcription factors, including nuclear factor (NF)-kB.9 This could explain the constitutive, as well as inducible, expression of NGAL in several of the non-hematopoietic tissues. Like other lipocalins, NGAL forms a barrel-shaped tertiary structure with a hydrophobic calyx that binds small lipophilic molecules.9 NGAL structure is depicted in Figure 1. Functional roles of NGAL The major ligands for NGAL are siderophores, small ironbinding molecules. Siderophores are synthesized by bacteria to acquire iron, and NGAL exerts a bacteriostatic effect by depleting siderophores. Therefore, NGAL is considered a critical component of innate immunity to bacterial infection. Experimental evidence for this role is derived from mice that are genetically modified to lack the NGAL gene, which renders them more susceptible to Gram-negative bacterial infections and death from sepsis.7 On the other hand, siderophores produced by eukaryotes participate in NGALmediated iron shuttling that is critical to various cellular responses, such as proliferation and differentiation.7–9 This property provides a potential molecular mechanism for the documented role of NGAL in enhancing the epithelial phenotype. During kidney development, NGAL promotes epithelial differentiation of the mesenchymal progenitors, leading to the generation of glomeruli, proximal tubules, Henle’s loop and distal tubules. Furthermore, although NGAL is expressed only at very low levels in several human tissues, it is markedly induced in injured epithelial cells, including the kidney. This is likely mediated via NF-kB, which is known to be rapidly activated in kidney tubule cells after acute injuries, and plays a central role in controlling cell survival and proliferation.10 NGAL induction in the context of AKI leads

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Figure 2. NGAL role in cellular physiology may be dependent on the type of molecule it is complexed with. Note: Apo-NGAL: NGAL that is devoid of siderophore or iron; ECM: extracellular matrix; VEGF: vascular endothelial growth factor; MMP-9: matrix metalloproteinase-9.

to marked preservation of function, attenuation of apoptosis and an enhanced proliferative response. This protective effect is dependent on the chelation of toxic iron from extracellular environment and the regulated delivery of siderophore and iron to intracellular sites.10 Additionally, NGAL is markedly induced in a number of human cancers, where it often represents a predictor of poor prognosis.7 The NGAL gene is known to be induced by a number of tumor-promoting agents, including SV40 and polyoma virus, phorbol esters, hepatocyte growth factor, retinoic acid, glucocorticoids and NF-kB.11 The overexpressed NGAL protein binds to matrix metalloproteinase (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 VEGF, and thus enabling angiogenesis, invasion and metastasis.11 A potentially unifying hypothesis to reconcile these seemingly contradictory roles of NGAL in human biology is offered in Figure 2. The mechanisms for the intracellular uptake of NGAL via receptors such as megalin, and for intracellular trafficking via endosomes have been elucidated.7 The subsequent molecular pathway taken by NGAL may be largely dependent on the type of molecule it is complexed with. Indeed, NGAL that is devoid of siderophore and iron (Apo-NGAL) rapidly scavenges intracellular iron. The resultant intra-cellular iron depletion results in a decrease in the mammalian cell’s proliferative ability and induction of apoptosis. On the other hand, when NGAL is bound to siderophore and iron, there is a rapid release of iron with regulation of iron-dependent molecular pathways, and downstream induction of proliferation and epithelial transformation. Finally, when NGAL is complexed with MMP-9 instead, there is enhancement of the active MMP-9 pool with resultant upregulation of MMP-9’s well known proangiogenic and proinvasive properties. Sources of urinary and plasma NGAL Although plasma NGAL is freely filtered by the glomerulus, it is almost totally reabsorbed in the proximal tubules by

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efficient megalin-dependent endocytosis.10 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 (1000-fold) upregulation of NGAL mRNA in the thick ascending limb of Henle’s loop and the collecting ducts.6,10 The resultant synthesis of NGAL protein in the distal nephron and secretion into the urine appears to comprise the major fraction of urinary NGAL. The latter is a monomeric molecule, whereas urinary NGAL originating from neutrophils is dimeric. 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.6,10 Regarding plasma NGAL in AKI, the kidney itself does not appear to be a major source, since direct ipsilateral renal vein sampling after unilateral ischemia indicates that the NGAL synthesized in the kidney is not introduced efficiently into the circulation, but is abundantly present in the ipsilateral ureter.10 However, it is now well known that AKI results in a dramatically increased NGAL mRNA expression in distant organs,6,10 especially the liver and lungs, and the overexpressed 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.6 Furthermore, any decrease in glomerular filtration rate (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. NGAL in the prediction of AKI Preclinical transcriptome profiling studies identified NGAL to be one of the most upregulated genes in the kidney very early after acute injury in animal models.12 Downstream proteomic analyses also revealed NGAL to be one of the most highly induced proteins in the kidney after ischemic or nephrotoxic AKI in animal models.13 The incidental 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 etiologies, a marked increase in urine and serum NGAL was documented by western blotting when compared with normal controls.14 Urine and serum NGAL levels correlated with serum creatinine and kidney biopsies in subjects with AKI who demonstrated intense accumulation of immunoreactive NGAL in cortical tubules, confirming NGAL as a sensitive index of established AKI in humans.7 A number of subsequent studies have now implicated NGAL as a promising,

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Table 2. NGAL as an AKI biomarker. NGAL properties Non-invasive, rapid, inexpensive method in urine or blood Sensitive for early diagnosis Amenable to existing clinical assay platforms High gradient to allow risk stratification Specific to AKI (AKI versus CKD vs. Systemic Disease)* Differential diagnosis between pre-renal azotemia and AKI The increase is proportional to injury or loss of renal function Associated with a clear pathophysiological mechanism Results predict clinical outcomes and treatment efficacy

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Note: Plasma NGAL may be detected in chronic kidney disease (CKD), chronic hypertension, systemic infections, inflammatory conditions and malignancies.6,7 Urine NGAL may be detected in CKD, lupus nephritis and urinary tract infections.6,7 In all these situations, NGAL values are generally substantially blunted compared to those typically measured in AKI.

non-invasive, troponin-like, early diagnostic biomarker for AKI in various common clinical settings.5–7 NGAL appears to fulfill many characteristics of an ideal AKI biomarker as demonstrated in Table 2. NGAL in CIN Several investigators have examined the role of plasma and urine NGAL as a predictive biomarker of CIN in adults and children undergoing coronary angiography with intra-arterial contrast administration and revealed good diagnostic and prognostic performance.15–20 In a prospective study of 91 children with congenital heart disease undergoing elective cardiac catheterization and angiography with contrast administration, both urine and plasma NGAL measured by using a research-based enzyme-linked immunosorbent assay (ELISA) at 2-h post-procedure predicted CIN, defined as a 50% increase in SCr from baseline, with an area under the receiver-operating characteristic curve (AUC-ROC) of 0.91– 0.92.18 Furthermore, pilot studies in adults with normal SCr receiving contrast for coronary angiography and intervention detected a significant rise in both urine and serum NGAL 2–4 h post-procedure, whereas serum cystatin C levels increased significantly only 24 h after contrast exposure.15 However, none of these subjects developed CIN, as SCr remained unchanged 48-h post-procedure.15 An extension of the same study in 100 patients confirmed these results and revealed CIN incidence of 11%.16 A study from China detected CIN in 8.7% of adults with normal renal function undergoing coronary angiography and found that urinary NGAL measured by a research-based assay at 24-h postprocedure increased significantly in the CIN group, but not in the non-CIN group.17 ROC curve analysis demonstrated that urinary NGAL at 24 h showed a good performance in early diagnosis of CIN as compared to SCr (AUC-ROC 0.73). Another study from Egypt showed a significant increase in serum NGAL 4 and 24 h after coronary interventions among adults with normal SCr.19 CIN, defined as a 25% increase in SCr from baseline, was detected in 6.7% of the participants. Finally, a meta-analysis revealed an overall AUC-ROC of 0.894 for CIN prediction, when plasma or urine NGAL was measured within 6 h after contrast administration for coronary procedures.20 A comparative presentation of the

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above-mentioned studies is provided, among others, in Table 3. Much of these above-mentioned data have been arbitrarily extrapolated to patients receiving intravenous (IV) contrast for CT scans despite major differences in patient populations, contrast volume administered and intra-procedural complications between the two settings.21 It is currently unclear how far the conclusions of such studies can be extrapolated to patients receiving IV contrast for CT scans. Furthermore, although the number of coronary and conventional angiograms has been relatively stable, the number of contrastenhanced CT scans has been increasing by approximately 2 million per year in the United States over the last decade.22 Of note, 90% of contrast media are used for CT imaging.22,23 Therefore, the widespread use of contrast-enhanced CT necessitates studies focusing on NGAL performance as an early and accurate AKI biomarker in this setting.23 NGAL in CIN after IV contrast for CT scan Our group designed a study to prospectively evaluate plasma NGAL measured by using standardized, point-of-care SriageÕ NGAL device (Biosite Incorporated, San Diego, CA)24 for early CIN detection among hospitalized patients undergoing elective contrast-enhanced CT.25 CIN, defined by an increase in SCr of 425% or 40.5 mg/dL from baseline within 48 h after contrast exposure, was found in 8.5% of subjects, but detection by SCr was possible 24–48 h postprocedure. In contrast, significant elevation of plasma NGAL by nearly 10-fold was found at 6-h post-procedure in those with versus those without CIN, with excellent performance characteristics of 6-h plasma NGAL for CIN prediction (area under the ROC curve of 1.00 with 95% CI of 0.92–1.00 at the cut-off value of 200 ng/mL). In summary, our study demonstrated that plasma NGAL 6 h after contrast administration measured by the rapid, pointof-care SriageÕ NGAL test appeared to be a clinically useful biomarker in the early prediction of CIN in hospitalized patients undergoing elective conventional contrast-enhanced CT. Therefore, plasma NGAL appears to be a powerful early biomarker of CIN that precedes the increase in SCr by several hours. Furthermore, the relatively high CIN incidence detected in this well-controlled population underlines the importance of early diagnosis by an adequate and simple procedure as the 6-h plasma SriageÕ NGAL test that was evaluated in our study. Our study is the first one that evaluates plasma NGAL performance for early CIN detection specifically after IV contrast administration. Our population consisted of subjects with well-preserved renal function and, thus, at lowto-medium risk of CIN. The predictive performance of NGAL varies with baseline renal function, and appears to be optimal in subjects with normal pre-procedure renal function.26 Furthermore, our design involved only elective CT scans, excluding emergent ones usually performed in subjects with major comorbidities and potentially at higher risk of CIN.27,28 The magnitude of rise by nearly 10-fold from baseline supports previous literature data that NGAL is a highly discriminatory biomarker with a wide dynamic range and cutoff values that allow for risk assessment and stratification.5,6

2007

2008 2010 2013 2013

2006 2009 2008 2008 2007 2008 2010 2010 2010

Hirsch et al.18*

Ling et al.17 Shaker et al.19 Filiopoulos et al.25 Lacquaniti et al.35

Nakamura et al.38 Bachorzewska-Gajewska et al.41 Bulent Gul et al.39 Ling et al.17 Bachorzewska-Gajewska et al.16 Kato et al.40 Briguori et al.42 Ishibashi et al.43 Shaker et al.19 L-FABP L-FABP IL-18 IL-18 CysC CysC CysC CysC CysC

NGAL NGAL NGAL NGAL

NGAL

NGAL

Biomarker

IA IA IA IA IA IA IA IA IA

IA IA IV IV

IA

IA

Administered contrast U S U P U S P S U U U U U S S S S S

U/P/S

Monoclonal Absa ELISAa ELISAa ELISAa Nephelometric immunoassaya Nephelometric immunoassaya Nephelometric immunoassaya Nephelometric immunoassaya NA

ELISAa NA ELISAb NA

ELISAa

ELISAa

Biomarker assay

SCr 40.5 mg/dL or 425% SCr 425% in 48 h SCr 40.5 mg/dL or 425% SCr 40.5 mg/dL or 425% SCr 425% within 48 h SCr 40.5 mg/dL or 425% SCr 0.3 mg/dL 48 h PP or SCr 40.5 mg/dL or 425% SCr 425%

within 48 h RRT within 48 h

within 2–5 days

SCr 40.5 mg/dL or 425% SCr 425% SCr 40.5 mg/dL or 425% within 48 h NA

SCr 450%

SCr 425% within 48 h

CIN definition

66/13 25/0 157/15 150/13 100/11 87/18 410/34 100/18 30/2

150/13 30/2 47/4 120/NA

91/11

100/11

Patients/ Events

NA NA 0.92 0.91 0.73 NA 1.00 0.99 0.99 NA NA NA 0.75 NA 0.93 0.92 40.50 NA

AUC-ROC

NA NA 100 82 20 NA 100 NA NA NA NA NA 20 NA 63 39 68 NA

PPV (%)

NA NA 96 89 97 NA 100 NA NA NA NA NA 96 NA 98 100 96 NA

NPV (%)

Notes: IA: intra-arterial, IV: intravenous, U: urine, P: plasma, S: serum, CIN: contrast-induced nephropathy, AUC-ROC: area under the receiver-operating characteristic curve, PPV: positive predictive value, NPV: negative predictive value, Abs: antibodies, SCr: serum creatinine, NA: not applicable or not available, NGAL: neutrophil gelatinase-associated lipocalin, ELISA: enzyme-linked immunosorbent assay, L-FABP: Liver-type fatty acid-binding protein, IL-18: interleukin 18, CysC: cystatin C, PP: post-procedure, RRT: renal replacement therapy. a Research-based assay, bStandardized clinical laboratory platform (Triage kit [Biosite, San Diego, CA] or ARCHITECT platform [Abbott, Abbott Park, IL]). *Study conducted in children.

2007

Publication year

Bachorzewska-Gajewska et al.16

Study

Table 3. Studies evaluating performance of NGAL and other biomarkers in CIN.

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The reported incidence of CIN varies widely depending on the presence or absence of risk factors, in particular preexisting CKD, the amount and type of agent administered and the exact radiologic procedure.28 Current data coming mainly from patients undergoing coronary angiography with intra-arterial contrast administration indicated that CIN occurs in 4–20% of cases.27,28 By contrast to that associated with angiography, the risk of CIN associated with contrastenhanced CT scans is quite low, even among patients with CKD.23,29 Therefore, in a study of in- and out-patients with eGFR less than 60 mL/min/1.73 m2 undergoing elective contrast-enhanced CT, only 3.5% had an increase in SCr of greater than 0.5 mg/dL (44 mmol/L) within 48 to 96 h postprocedure.29 In addition, three recent prospective trials involving contrast-enhanced CT in patients with eGFR less than 60 mL/min/1.73 m2 found an overall incidence of CIN, defined as a 25% increase in SCr, of approximately 5%.30–32 In view of these data, CIN incidence of 8.5% reported in our study and others appears to be relatively high. However, above-mentioned differences in study populations and variations in study design and methodology (e.g., assay used for NGAL measurement) should be taken into account in the interpretation of the results. Previous studies on NGAL performance as predictive biomarker of CIN used mainly research-based ELISA assays for plasma NGAL measurement,15–20 which are not practical in the clinical setting. In this regard, a major advance has been the development of a standardized, point-of-care SriageÕ NGAL test (Biosite Incorporated, San Diego, CA) for plasma NGAL measurement.24 A potential limitation is the suboptimal assay performance in the lower-range values. However, a meta-analysis on the accuracy of NGAL in the diagnosis of AKI revealed better performance of standardized assays compared with individually developed research-based ones.20 SriageÕ NGAL device, although validated in several clinical settings with favorable results,24 had not been evaluated so far in subjects undergoing contrast-enhanced CT scans. Our study25 supports its use in the latter clinical setting as well. With respect to the sample source, the majority of biomarkers for AKI described to date have been determined in the urine.5,6 NGAL urinary diagnostics do have several advantages, but also some important limitations compared to plasma/serum NGAL (Table 4). Indeed, urine NGAL levels were correlated with GFR in subjects with CKD due to glomerulonephritis, as well as in those with autosomal dominant polycystic kidney disease.6,33 Furthermore, urinary NGAL may be increased in urinary tract infections.6

Simultaneously, plasma or serum NGAL measurements may be influenced by a number of co-existing variables such as CKD, hypertension, systemic infections, inflammatory conditions and malignancies.4–7 In case of intra-arterial coronary angiography and intervention, NGAL might be released from atherosclerotic plaques and, subsequently, increase plasma pool, in the absence of co-existing AKI.6,7,21,22 However, the increase in both plasma/serum and urine NGAL in the above-mentioned situations is generally much less than that typically encountered in AKI. In any case, plasma-based diagnostics have revolutionized many aspects of clinical medicine, as exemplified by the use of troponins for the early diagnosis of acute myocardial infarction and the value of B-type natriuretic peptide for prognostication in acute coronary syndromes.5,6,34 Another study on NGAL performance in CIN after exposure to IV contrast material has been published almost simultaneously to ours.35 Both serum and urine NGAL showed excellent performance in predicting CIN, 8 h after iodinated contrast administration (AUC-ROC 0.995 and 0.992 for serum and urine NGAL, respectively), while SCr level changes occurred 24-h post-procedure. Despite differences in study design and participants, these results appear to be in line with our own findings, providing further support to plasma/serum NGAL role as an early predictive biomarker of CIN after IV contrast administration. A comparative presentation of studies evaluating NGAL performance in CIN is provided in Table 3. Apart from NGAL, several other biomarkers, such as cystatin C, Interleukin 18 (IL-18) and liver-type fatty acidbinding protein (L-FABP), have been evaluated in the prediction of CIN, with inconsistent results due, mainly, to differences in study populations, design and clinical settings.36–43 NGAL, both in plasma and urine, shows superior performance as early CIN biomarker.36,37 Indeed, it was pronounced by experts in the field as the ‘‘renal troponin’’.6 A brief comparative presentation of studies on biomarkers for the early prediction of CIN is shown in Table 3. However, results on biomarkers’ performance should always be interpreted with caution, in particular, when validated in heterogenic populations with co-morbid conditions, such as diabetes mellitus, vascular disease and CKD.

Summary Over the last decade there has been considerable progress in the discovery and development of biomarkers of AKI, and several has now been evaluated in various clinical settings.

Table 4. Urinary versus plasma/serum NGAL diagnostics. Pro Urine

 Non-invasive nature of sample collection  Reduced number of interfering proteins  Potential for development of self-testing kits

Plasma/Serum

 Sample available even in case of oligoanuria  Not influenced by hydration status or diuretic therapy or by co-existing urinary tract infection or proteinuria

Contra  Lack of sample in case of oligoanuria  NGAL concentration may be influenced by hydration status or diuretic therapy or by co-existing urinary tract infection, as well as by co-existing CKD or in case of proteinuria  Invasive nature of sample collection  NGAL concentration may be influenced by co-existing CKD, hypertension, systemic infection, inflammatory conditions and malignancies

Note: NGAL: neutrophil gelatinase-associated lipocalin, CKD: chronic kidney disease.

NGAL in contrast nephropathy after CT

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DOI: 10.3109/0886022X.2014.900429

NGAL as a promising 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 standardized laboratory platforms, in an attempt to identify the best biomarkers for each purpose (risk assessment, diagnosis, determination of cause for differential diagnosis and prognosis). CIN is a common cause of hospital-acquired AKI and a source of substantial morbidity and mortality. Several biomarkers have been evaluated in the early prediction of CIN, not always with robust results. The application of such biomarkers in clinical practice remains controversial, especially for populations with major co-morbidities. However, NGAL, both in plasma/serum and urine, appears to fulfill many characteristics of an ideal early AKI biomarker. Its role in the timely diagnosis of CIN has already been examined in adults and children undergoing coronary angiography and a meta-analysis revealed a very good performance of plasma or urine NGAL, measured within 6 h after contrast administration, for CIN early diagnosis. However, limited information exists on NGAL performance in CIN after IV contrast administration for CT scans. A recently published study from our group offered some data on this understudied area. Therefore, plasma NGAL measured by a rapid, standardized assay appears to be a powerful early predictive biomarker for CIN among the important group of hospitalized patients undergoing elective contrast-enhanced CT. Our results could serve as a useful working hypothesis for further studies with a larger number of patients to demonstrate the potential role of NGAL for CIN prediction in this population. Although growing body of evidence suggests that the incidence of CIN may be less with IV contrast administration than that reported in coronary diagnostic procedures, the widespread use of contrast-enhanced CT examinations still leaves a sizeable population vulnerable to this event. An important incidence of CIN was confirmed in the wellcontrolled population of our study making an early diagnostic tool even more desirable. Indeed, an increase in plasma NGAL would potentially trigger clinicians to monitor patients more closely, avoid treatment with additional nephrotoxins, and optimize hydration and renal perfusion to prevent further injury, limit re-hospitalization and, subsequently, reduce CTassociated morbidity and cost. However, our ability of early intervention in the evolution of CIN was limited up to now by the lack of an early, appropriately sensitive and specific marker of kidney injury. Therefore, timely detection of imminent CIN by an accurate and easy-to-perform procedure as the standardized, point-of-care plasma SriageÕ NGAL test might enable early initiation of interventions or, at least, promote increased vigilance to improve the mid-term adverse outcomes associated with this rather common clinical complication.

Declaration of interest The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

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