Postoperative biomarkers predict acute kidney injury and poor outcomes after adult cardiac surgery

[Biomarkers of acute kidney injury: a « trending topic » in cirrhosis]

Acute kidney injury (AKI) is an ominous event in the natural history of cirrhosis. The differential diagnosis of this entity is hampered by the absence of specific biomarkers of tubular damage in cirrhosis. The clinical usefulness of such biomarkers is determined by their effectiveness in the diagnosis of AKI and their ability to provide critical information to ameliorate clinical outcomes and survival. The lack of biomarkers has hindered the development of interventions aimed to improve the prognosis of kidney impairment in cirrhosis. Currently, biomarkers are an area of intense research in nephrology. Emerging genomic and proteomic technologies have revealed novel plasma and urinary biomarkers of AKI. The present article discusses the most promising candidate biomarkers with potential application in cirrhosis, such as NGAL, KIM-1, cystatin-C, IL-18, L-FABP, N-acetyl glucosaminidase and netrin-1, are discussed below.

The clinical utility of plasma neutrophil gelatinase-associated lipocalin in acute kidney injury

BACKGROUND AND AIM

Neutrophil gelatinase-associated lipocalin (NGAL) is derived from the distal tubule and is both reabsorbed and filtered and also shed into the urine after tubular injury. Plasma NGAL is unique amongst the candidate biomarkers of acute kidney injury (AKI) since elevated concentrations may reflect either a change in renal glomerular function or in structural tubular injury or both. In this study, we compared the performance of plasma NGAL in the diagnosis of functional changes and in the diagnosis of structural injury.

METHODS

Plasma and urine samples from 528 patients were collected on entry to an intensive care unit (ICU) as well as 12 and 24 h later. Plasma NGAL diagnostic performance was independently assessed for Functional-AKI and Structural-AKI. Functional-AKI was defined by changes in plasma creatinine, whereas Structural-AKI was defined by elevations in urinary NGAL.

RESULTS

On ICU entry, the area under the curve (AUC) for the diagnosis of Functional-AKI was 0.74 (95% CI: 0.69-0.79), and for Structural-AKI it was 0.79 (0.74-0.83). Plasma NGAL also predicted the need for dialysis (0.79; 0.66-0.81), but not for death. A principal component analysis demonstrated that the maximum plasma NGAL in 24 h reflected structural injury marginally more than functional changes. Plasma NGAL added value to an AKI diagnostic model comprising plasma creatinine, sepsis, age, and APACHE II score (integrated discrimination improvement: 0.073; 0.034-0.12).

CONCLUSION

Increased plasma NGAL reflects both decreased filtration and structural injury. For patients at a low calculated risk, the addition of NGAL reduced the risk, and for those at a higher risk, NGAL correctly assigned patients to even a higher risk.

Distinct injury markers for the early detection and prognosis of incident acute kidney injury in critically ill adults with preserved kidney function

The use of novel biomarkers to detect incident acute kidney injury (AKI) in the critically ill is hindered by heterogeneity of injury and the potentially confounding effects of prevalent AKI. Here we examined the ability of urine NGAL (NGAL), L-type Fatty Acid Binding Protein (L-FABP), and Cystatin C to predict AKI development, death, and dialysis in a nested case-control study of 380 critically ill adults with an eGFR over 60 ml/min/1.73 m². One-hundred thirty AKI cases were identified following biomarker measurement and were compared to 250 controls without AKI. Areas under the receiver-operator characteristic curves (AUC-ROCs) for discriminating incident AKI from non-AKI were 0.58(95%CI: 0.52-0.64), 0.59(0.52-0.65), and 0.50(0.48-0.57) for urine NGAL, L-FABP, and Cystatin C, respectively. The combined AUC-ROC for NGAL and L-FABP was 0.59(56-0.69). Both urine NGAL and L-FABP independently predicted AKI during multivariate regression; however, risk reclassification indices were mixed. Neither urine biomarker was independently associated with death or acute dialysis [NGAL hazard ratio 1.35(95%CI: 0.93-1.96), L-FABP 1.15(0.82-1.61)] though both independently predicted the need for acute dialysis [NGAL 3.44(1.73-6.83), L-FABP 2.36(1.30-4.25)]. Thus, urine NGAL and L-FABP independently associated with the development of incident AKI and receipt of dialysis but exhibited poor discrimination for incident AKI using conventional definitions.

Performance of kidney injury molecule-1 and liver fatty acid-binding protein and combined biomarkers of AKI after cardiac surgery

BACKGROUND AND OBJECTIVES

AKI is common and novel biomarkers may help provide earlier diagnosis and prognosis of AKI in the postoperative period.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This was a prospective, multicenter cohort study involving 1219 adults and 311 children consecutively enrolled at eight academic medical centers. Performance of two urine biomarkers, kidney injury molecule-1 (KIM-1) and liver fatty acid-binding protein (L-FABP), alone or in combination with other injury biomarkers during the perioperative period was evaluated. AKI was defined as doubling of serum creatinine or need for acute dialysis.

RESULTS

KIM-1 peaked 2 days after surgery in adults and 1 day after surgery in children, whereas L-FABP peaked within 6 hours after surgery in both age groups. In multivariable analyses, the highest quintile of the first postoperative KIM-1 level was associated with AKI compared with the lowest quintile in adults, whereas the first postoperative L-FABP was not associated with AKI. Both KIM-1 and L-FABP were not significantly associated with AKI in adults or children after adjusting for other kidney injury biomarkers (neutrophil gelatinase-associated lipocalin and IL-18). The highest area under the curves achievable for discrimination for AKI were 0.78 in adults using urine KIM-1 from 6 to 12 hours, urine IL-18 from day 2, and plasma neutrophil gelatinase-associated lipocalin from day 2 and 0.78 in children using urine IL-18 from 0 to 6 hours and urine L-FABP from day 2.

CONCLUSIONS

Postoperative elevations of KIM-1 associate with AKI and adverse outcmes in adults but were not independent of other AKI biomarkers. A panel of multiple biomarkers provided moderate discrimination for AKI.

Neutrophil gelatinase-associated lipocalin: pathophysiology and clinical applications

Neutrophil gelatinase-associated lipocalin (NGAL), a 25 kDa protein produced by injured nephron epithelia, is one of the most promising new markers of renal epithelial injury. In contrast to serum creatinine and urinary output, which are the measures of kidney function, NGAL is specifically induced in the damaged nephron and then released into blood and urine, where it can be readily measured. Careful proof-of-concept studies using defined animal models have uncovered the sources and trafficking of NGAL in acute kidney injury (AKI) and have addressed the contributions of renal and non-renal sources. Clinical studies indicate that NGAL, unlike creatinine, is a marker responsive to tissue stress and nephron injury, but less so to adaptive hemodynamic responses. In certain clinical settings, NGAL is an earlier marker compared with serum creatinine. In addition, clinical studies have shown that NGAL is a powerful predictor of poor clinical outcomes, which can be used to risk stratify patients when combined with serum creatinine. NGAL has important limitations, including its responsiveness in systemic inflammation, which is partially uncoupled from its response to kidney injury and which needs to be considered when interpreting NGAL results clinically. This review covers the biology and pathophysiology of NGAL and summarizes the results of the growing body of clinical studies that have addressed the utility of NGAL in the early diagnosis of AKI, in the distinction of intrinsic AKI and in the prognostic assessment of broad patient populations.

Neutrophil gelatinase-associated lipocalin (NGAL) levels in response to unilateral renal ischaemia in a novel pilot two-kidney porcine model

OBJECTIVES

To test a novel porcine two-kidney model for evaluating the effect of controlled acute kidney injury (AKI) related to induced unilateral ischaemia on both renal units (RUs) To use neutrophil gelatinase-associated lipocalin (NGAL) and physiological serum and urinary markers to assess AKI and renal function.

METHODS

Twelve female Yorkshire pigs had bilateral cutaneous ureterostomies placed laparoscopically with identical duration of pneumoperitoneum for all cases. An experimental group (n = 9) underwent induced unilateral renal ischaemia with left hilar clamping of timed duration (15, 30, 60 min) and a control group (n = 3) had no induced renal ischaemia. Urine was collected and analysed from each RU to assess creatinine and NGAL concentration preoperatively and at multiple postoperative time points. Serum was collected and analysed daily for creatinine and NGAL levels. Statistical comparisons were made using the rank-sum and sign-rank tests.

RESULTS

Three pigs were excluded because of intra-operative and postoperative complications. In the RUs that experienced renal ischaemia (n = 7),the median urine volume was lower (P = 0.04) at 6, 12, 24 and 48 h and the median NGAL concentration was higher (P = 0.04) at 12 and 48 h compared with the RUs of control pigs that experienced no renal ischaemia (n = 2). When comparing the ischaemic (left) RU of the pigs in the experimental group with their contralateral non-ischaemic (right) RU, ischaemic RUs had a lower median cumulative urine volume at 6, 12, 24 and 48 h (P = 0.05) and a higher median NGAL concentration at 12, 24 and 48 h (P < 0.05). At 48 h, no significant increase was found in serum NGAL in pigs in the experimental group compared with controls (P = 0.2). Creatinine clearance (CC) was lower in ischaemic RUs compared with non-ischaemic RUs 1 day after surgery (P = 0.04) with decreasing CC as the duration of ischaemia increased.

CONCLUSIONS

We have developed a promising novel small-scale pilot surgical model that allowed the evaluation of bilateral RU function separately during and after unilateral renal ischaemia. The induction of unilateral renal ischaemia corresponds with physiological changes in both the ischaemic and contralateral RU. AKI as measured by increases in NGAL and decreased renal function as measured by decreases in CC, are specific to the RU exposed to ischaemia.

Clinical utility of biomarkers of AKI in cardiac surgery and critical illness

AKI is a common and serious complication that is associated with several adverse outcomes in hospitalized patients. The past several years have seen a large number of multicenter investigations of biomarkers of AKI in the setting of cardiac surgery and critical illness. This review summarizes these biomarker results to identify applications for clinical use. The Translational Research Investigating Biomarker Endpoints in AKI (TRIBE-AKI) study showed that blood and urine biomarkers measured preoperatively, immediately postoperatively, and at the time of the clinical increase in serum creatinine in the setting of cardiac surgery all had the ability to improve patient risk stratification for a variety of important clinical end points. Analyses of biomarkers concentrations from the Acute Respiratory Distress Syndrome Network, EARLY ARF, and other studies of critically ill subjects have similarly shown that biomarkers measured early in the clinical course can forecast the development of AKI and need for renal replacement therapy as well as inpatient mortality. Although biomarkers have informed the diagnosis, prognosis, and treatment of AKI and are inching closer to clinical application, large multicenter interventional clinical trials to prevent AKI using biomarkers should continue to be an active area of clinical investigation.

Fluid management in cardiac surgery: colloid or crystalloid?

The crystalloid-colloid debate has raged for decades, with the publication of many meta-analyses, yet no consensus. There are important differences between colloids and crystalloids, and these differences have direct relevance for cardiac surgical patients. Rather than asking crystalloid or colloid, we believe better questions to ask are (1) High or low chloride content? and (2) Synthetic or natural colloid? In this paper we review the published literature regarding fluid therapy in cardiac surgery and explain the background to these two important and unanswered questions.

Role of new biomarkers: functional and structural damage

Traditional diagnosis of acute kidney injury (AKI) depends on detection of oliguria and rise of serum creatinine level, which is an unreliable and delayed marker of kidney damage. Delayed diagnosis of AKI in the critically ill patient is related to increased morbidity and mortality, prolonged length of stay, and cost escalation. The discovery of a reliable biomarker for early diagnosis of AKI would be very helpful in facilitating early intervention, evaluating the effectiveness of therapy, and eventually reducing cost and improving outcome. Innovative technologies such as genomics and proteomics have contributed to the discovery of new biomarkers, such as neutrophil gelatinase-associated lipocalin (NGAL), cystatin C (Cys C), kidney injury molecule-1 (KIM-1), interleukin-18 (IL-18), and liver-type fatty acid binding protein (L-FABP). The current status of the most promising of these novel AKI biomarkers, including NGAL, Cys C, KIM-1, L-FABP, and IL-18, is reviewed.

Urinary markers of kidney injury and kidney function decline in HIV-infected women

OBJECTIVE

HIV-infected persons have substantially higher risk of kidney failure than persons without HIV, but serum creatinine levels are insensitive for detecting declining kidney function. We hypothesized that urine markers of kidney injury would be associated with declining kidney function among HIV-infected women.

METHODS

In the Women's Interagency HIV Study, we measured concentrations of albumin-to-creatinine ratio, interleukin-18 (IL-18), kidney injury marker-1 (KIM-1), and neutrophil gelatinase-associated lipocalin from stored urine among 908 HIV-infected and 289 HIV-uninfected participants. Primary analyses used cystatin C-based estimated glomerular filtration rate (CKD-EPI eGFRcys) as the outcome, measured at baseline and 2 follow-up visits over 8 years; secondary analyses used creatinine (CKD-EPI eGFRcr). Each urine biomarker was categorized into tertiles, and kidney decline was modeled with both continuous and dichotomized outcomes.

RESULTS

Compared with the lowest tertiles, the highest tertiles of albumin-to-creatinine ratio (-0.15 mL/min per 1.73 m, P < 0.0001), IL-18 (-0.09 mL/min per 1.73 m, P < 0.0001) and KIM-1 (-0.06 mL/min per 1.73 m, P < 0.001) were independently associated with faster eGFRcys decline after multivariate adjustment including all 3 biomarkers among HIV-infected women. Among these biomarkers, only IL-18 was associated with each dichotomized eGFRcys outcome: ≥3% (relative risk = 1.40; 95% confidence interval: 1.04 to 1.89); ≥5% (1.88; 1.30 to 2.71); and ≥10% (2.16; 1.20 to 3.88) for the highest versus lowest tertile. In alternative models using eGFRcr, the high tertile of KIM-1 had independent associations with 5% (1.71; 1.25 to 2.33) and 10% (1.78; 1.07 to 2.96) decline, and the high IL-18 tertile with 10% decline (1.97; 1.00 to 3.87).

CONCLUSIONS

Among HIV-infected women in the Women's Interagency HIV Study cohort, novel urine markers of kidney injury detect risk for subsequent declines in kidney function.

Urine IL-18, NGAL, IL-8 and serum IL-8 are biomarkers of acute kidney injury following liver transplantation

BACKGROUND

AKI is common following liver transplantation and is associated with significant morbidity and mortality. Biomarkers of AKI have not been well established in this setting but are needed to help guide patient care and facilitate development of novel therapeutics.

METHODS

Serum creatinine, cystatin C, IL-6, and IL-8 and urine IL-18, NGAL, IL-6, and IL-8 were measured before and within 24 hours after liver transplantation in 40 patients. AKI was defined as a ≥50% sustained increase in creatinine above pre-operative values occurring within 24 hours of transplantation and persisting for at least 24 hours.

RESULTS

Seven patients met criteria for AKI (17.5%), with mean creatinines of 0.81 mg/dL pre-operatively and 1.75 mg/dL post-operatively. While pre-operative biomarker levels in patients with AKI were similar to those in patients without AKI, differences were seen between the groups with regard to median post-operative serum IL-8 (pg/mL) (242.48 vs. 82.37, p = 0.0463) and urine NGAL (ng/mL) (386.86 vs. 24.31, p = 0.0039), IL-6 (pg/mL) (52 vs. 7.29, p=0.0532), IL-8 (pg/mL) (14.3 vs. 0, p = 0.0224), and IL-18 (pg/mL) (883.09 vs. 0, p = 0.0449). The areas under receiver operating characteristic (ROC) curves were 0.749 for urine IL-18, 0.833 for urine NGAL, 0.745 for urine IL-6, 0.682 for serum IL-6, 0.773 for urine IL-8, and 0.742 for serum IL-8. Post-operative cystatin C was not significantly different between AKI and no AKI groups.

CONCLUSION

Serum IL-8 and urine IL-18, NGAL, IL-6, and IL-8 are elevated in AKI within the first 24 hours following liver transplantation.

Urinary cystatin C and acute kidney injury after cardiac surgery

BACKGROUND

Acute kidney injury (AKI) is common after cardiac surgery and is associated with adverse patient outcomes. Urinary cystatin C (CysC) level is a biomarker of proximal tubule function and may increase earlier in AKI than serum creatinine level.

STUDY DESIGN

Prospective cohort study.

SETTINGS & PARTICIPANTS

The TRIBE AKI (Translational Research Investigating Biomarker Endpoints in AKI) Consortium prospectively enrolled 1,203 adults and 299 children and adolescents at 8 institutions in 2007-2009.

INDEX TEST

Urinary CysC (in milligrams per liter) within the first 12 hours after surgery.

OUTCOME

Serum creatinine-based AKI was defined as AKI Network stage 1 (mild AKI) and doubling of serum creatinine from the preoperative value or need for dialysis during hospitalization (severe AKI).

OTHER MEASUREMENTS

Analyses were adjusted for characteristics used clinically for AKI risk stratification, including age, sex, race, estimated glomerular filtration rate, diabetes, hypertension, heart failure, nonelective surgery, cardiac catheterization within 72 hours, type of surgery, myocardial infarction, and cardiopulmonary bypass time longer than 120 minutes.

RESULTS

Urinary CysC level measured in the early postoperative period (0-6 and 6-12 hours postoperatively) correlated with both mild and severe AKI in adults and children. However, after analyses were adjusted for other factors, the effect was attenuated for both forms of AKI in both cohorts.

LIMITATIONS

Limited numbers of patients with severe AKI and in-hospital dialysis treatment.

CONCLUSIONS

Urinary CysC values are not associated significantly with the development of AKI after cardiac surgery in adults and children.

Neutrophil gelatinase-associated lipocalin as a biomarker of cardiovascular disease: a systematic review

BACKGROUND

Neutrophil gelatinase-associated lipocalin (NGAL) is a biomarker of acute kidney injury (AKI). Recently, elevated NGAL levels have also been reported in heart failure, coronary heart disease, and stroke. Other studies demonstrate that NGAL is upregulated in failing myocardium and in atherosclerotic plaque. Our aim was to synthesize the current evidence on NGAL and cardiovascular disease (CVD), and to clarify the prognostic significance of systemic NGAL levels in CVD.

METHODS

We performed a systematic review to identify experimental and human studies on NGAL and CVD. We excluded articles which specifically dealt with AKI or renal endpoints.

RESULTS

We identified 22 studies, including both animal and human data. NGAL is highly expressed in the heart, both in failing myocardium and myocarditis, and is also expressed in atherosclerotic plaques. Areas of co-localization of NGAL and matrix metalloproteinase (MMP)-9 exhibited increased MMP-9 proteolytic activity. Systemic NGAL levels correlated with renal function and severity of CVD in several, but not all, studies. An association between elevated systemic NGAL levels and clinical outcomes (e.g., death, hospital readmissions) were reported in six CVD studies, but these had limited adjustment for potential confounders.

CONCLUSIONS

There is ample literature to support a putative role of NGAL in the pathophysiology of CVD, but at present there is insufficient data regarding the clinical utility of systemic NGAL levels in the management of CVD. Available evidence regarding NGAL as a predictor of outcomes in CVD is very limited.

Serum Neutrophil Gelatinase-Associated Lipocalin versus Serum Creatinine for the Prediction of Acute Kidney Injury after Liver Transplantation

BACKGROUND

Acute kidney injury (AKI) is a common complication after liver transplantation (LT), and considerably increases the morbidity and mortality of the procedure. The gold standard of measuring the kidney function, the serum creatinine level (sCr), has poor specificity and sensitivity for the early diagnosis of AKI. Novel biomarkers for the prediction or early diagnosis of AKI, would potentially increase the opportunities for therapeutic interventions.

OBJECTIVE

To compare the diagnostic value of the standard renal marker, sCr and neutrophil gelatinase-associated lipocalin (NGAL) to predict AKI within 48 hours of LT.

METHODS

During a 9-month period from 2010 to 2011, NGAL was measured two times in 90 patients who underwent LT from deceased donors-after induction of anesthesia (NGAL1) and 2 hours after reperfusion of the liver graft (NGAL2). Patients were grouped according to the presence of risk factors for developing AKI according to the Acute Kidney Injury Network criteria (increase of ≥0.3 mg/dL in plasma creatinine above the baseline value within 48 hours).

RESULTS

60 men and 30 women with mean±SD age of 40.2±14.2 years were included in this study. The incidence of AKI was 34% (95% CI: 24%-44%). The difference between the NGAL1 and NGAL2 (ΔNGAL) and the baseline NGAL concentration was predictive of AKI in all patients. Receiver operating characteristic (ROC) curve and area under curves (AUCs) of ΔNGAL and sCr levels during the first 48 hours of LT were similar in predicting AKI. The AUCs of the ΔNGAL to predict AKI was 0.64 (95% CI: 0.52-0.76). The development of AKI was significantly correlated with the number of units of fresh frozen plasma transfused intra-operatively (p=0.017) and cold ischemic time (p=0.042).

CONCLUSION

Serum NGAL concentrations obtained during surgery is a predictor of AKI in patients undergoing LT.

Chronic kidney disease following acute kidney injury-risk and outcomes

In the past two decades, a substantial increase in the incidence of acute kidney injury (AKI) and kidney injury requiring dialysis has occurred in North America. This increase has coincided with an increase in the incidence of end-stage renal disease (ESRD), which has exceeded that expected based upon the prevalence of chronic kidney disease (CKD). In order to better understand the association between these conditions, there has been a proliferation of studies that have examined the risks of incident and progressive CKD following AKI. Animal studies have shown that failed differentiation of epithelial cells following renal ischaemia-reperfusion injury might lead to tubulointerstitial fibrosis, supporting a biological mechanism linking AKI and CKD. Strong and consistent associations between AKI and incident CKD, progression of CKD and incident ESRD have also been shown in epidemiological studies. In this Review, we summarize the wealth of available data on the relationship between AKI and CKD, and discuss the implications of these findings for the long-term clinical management of patients following AKI. We also identify areas of active investigation and future directions for research.

The emerging role of Klotho in clinical nephrology

Klotho is highly expressed in the kidney and a soluble form of Klotho functions as an endocrine substance that exerts multiple actions including the modulation of renal solute transport and the protection of the kidney from a variety of insults in experimental models. At present, the Klotho database is still largely preclinical, but the anticipated forthcoming impact on clinical nephrology can be immense. This manuscript puts these potentials into perspective for the clinician. There is renal and systemic Klotho deficiency in both acute kidney injury (AKI) and chronic kidney disease (CKD). Klotho plummets very early and severely in AKI and represents a pathogenic factor that exacerbates acute kidney damage. In CKD, Klotho deficiency exerts a significant impact on progression of renal disease and extra renal complications. In AKI, soluble Klotho levels in plasma and/or urine may serve as an early biomarker for kidney parenchymal injury. Restoration by exogenous supplementation or stimulation of endogenous Klotho may prevent and/or ameliorate kidney injury and mitigate CKD development. In CKD, Klotho levels may be an indicator of early disease and predict the rate of progression, and presence and severity of soft tissue calcification. The correction of Klotho deficiency may delay progression and forestall development of extra renal complications in CKD. Rarely does one find a molecule with such broad potential applications in nephrology. Klotho can possibly emerge on the horizon as a candidate for an unprecedented sole biomarker and intervention. Nephrologists should monitor the progress of the preclinical studies and the imminently emerging human database.

Performance of urinary liver-type fatty acid-binding protein in acute kidney injury: a meta-analysis

BACKGROUND

Urinary liver-type fatty acid-binding protein (L-FABP) is a proximal tubular injury candidate biomarker for early detection of acute kidney injury (AKI), with variable performance characteristics depending on clinical settings.

STUDY DESIGN

Meta-analysis of diagnostic test studies assessing the performance of urinary L-FABP in AKI.

SETTING & POPULATION

Literature search in MEDLINE, EMBASE, Scopus, Google Scholar, Cochrane Central Register of Controlled Trials, and ClinicalTrials.gov using search terms "liver-type fatty acid-binding protein" and "L-FABP."

SELECTION CRITERIA FOR STUDIES

Studies of humans investigating the performance characteristics of urinary L-FABP for the early diagnosis of AKI and AKI-related outcomes, including dialysis requirement and mortality.

PREDICTOR

Urinary L-FABP.

OUTCOMES

Diagnosis of AKI, dialysis requirement, and in-hospital death.

RESULTS

15 prospective cohort and 2 case-control studies were identified. Only 7 cohort studies could be meta-analyzed. The estimated sensitivity of urinary L-FABP level for the diagnosis of AKI was 74.5% (95% CI, 60.4%-84.8%), and specificity was 77.6% (95% CI, 61.5%-88.2%). The estimated sensitivity of urinary L-FABP level for predicting dialysis requirement was 69.1% (95% CI, 34.6%-90.5%), and specificity was 42.7% (95% CI, 3.1%-94.5%); for in-hospital mortality, sensitivity and specificity were 93.2% (95% CI, 66.2%-99.0%) and 78.8% (95% CI, 27.0%-97.4%), respectively.

LIMITATIONS

Paucity and low quality of studies, different clinical settings, and variable definitions of AKI.

CONCLUSIONS

Although urinary L-FABP may be a promising biomarker for early detection of AKI and prediction of dialysis requirement and in-hospital mortality, its potential value needs to be validated in large studies and across a broader spectrum of clinical settings.

Advances in acute kidney injury associated with cardiac surgery: the unfolding revolution in early detection

Cardiac surgery-associated acute kidney injury (CSA-AKI) is important because it remains common and serious. A major limitation in the management of CSA-AKI has been ongoing delayed diagnosis by standard clinical approaches, including serum creatinine and calculated glomerular filtration rate. Recent advances in the understanding of CSA-AKI have highlighted the utility of novel biomarkers that diagnose CSA-AKI within the first 24 hours. The biomarkers that have been evaluated in clinical trials include neutrophil gelatinase-associated lipocalin (NGAL), cystatin C, kidney injury molecule 1 and interleukin-18. The biomarker with the greatest clinical promise is NGAL. Although it has multiple advantages over serum creatinine, it is still not the ideal biomarker for CSA-AKI. It is likely that a panel of early biomarkers will be developed to facilitate rapid and reliable detection of CSA-AKI, combining their different characteristics to optimize patient management. Future clinical trials likely will focus on whether these biomarkers predict adverse outcomes independent of serum creatinine fluctuations and whether therapies guided by biomarker profiles improve renal salvage and overall clinical outcomes. Given their clinical utility, these novel biomarkers have been evaluated beyond cardiac surgery for AKI in multiple clinical environments, including the emergency department, the operating room, the cardiac catheterization laboratory, and the intensive care unit. Their integration into clinical practice seems likely in the near future.

Urinary angiotensinogen and risk of severe AKI

BACKGROUND

Biomarkers of AKI that can predict which patients will develop severe renal disease at the time of diagnosis will facilitate timely intervention in populations at risk of adverse outcomes.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Liquid chromatography/tandem mass spectrometry was used to identify 30 potential prognostic urinary biomarkers of severe AKI in a group of patients that developed AKI after cardiac surgery. Angiotensinogen had the best discriminative characteristics. Urinary angiotensinogen was subsequently measured by ELISA and its prognostic predictive power was verified in 97 patients who underwent cardiac surgery between August 1, 2008 and October 6, 2011.

RESULTS

The urine angiotensinogen/creatinine ratio (uAnCR) predicted worsening of AKI, Acute Kidney Injury Network (AKIN) stage 3, need for renal replacement therapy, discharge >7 days from sample collection, and composite outcomes of AKIN stage 2 or 3, AKIN stage 3 or death, and renal replacement therapy or death. The prognostic predictive power of uAnCR was improved when only patients classified as AKIN stage 1 at the time of urine sample collection (n=79) were used in the analysis, among whom it predicted development of stage 3 AKI or death with an area under the curve of 0.81. Finally, category free net reclassification improvement showed that the addition of uAnCR to a clinical model to predict worsening of AKI improved the predictive power.

CONCLUSIONS

Elevated uAnCR is associated with adverse outcomes in patients with AKI. These data are the first to demonstrate the utility of angiotensinogen as a prognostic biomarker of AKI after cardiac surgery.

New biomarkers for the quick detection of acute kidney injury

Acute kidney injury (AKI) is a common and strong problem in the diagnosis of which based on measurement of BUN and serum creatinine. These traditional methods are not sensitive and specific for the diagnosis of AKI. AKI is associated with increased morbidity and mortality in critically ill patients and a quick detection is impossible with BUN and serum creatinine. A number of serum and urinary proteins have been identified that may messenger AKI prior to a rise in BUN and serum creatinine. New biomarkers of AKI, including NGAL, KIM-1, cystatin-C, IL-18, and L-FABP, are more favourable tests than creatinine which have been identified and studied in several experimental and clinical training. This paper will discuss some of these new biomarkers and their potential as useful signs of AKI. We searched the literature using PubMed and MEDLINE with acute kidney injury, urine, and serum new biomarkers and the articles were selected only from publication types in English.

Urinary and serum biomarkers for the diagnosis of acute kidney injury: an in-depth review of the literature

BACKGROUND

Acute kidney injury (AKI) remains associated with high morbidity and mortality, despite progress in medical care. Although the RIFLE (Risk, Injury, Failure, Loss, End-Stage Kidney Disease) and AKIN (Acute Kidney Injury Network) criteria, based on serum creatinine and urine output, were a step forward in diagnosing AKI, a reliable tool to differentiate between true parenchymal and pre-renal azotaemia in clinical practice is still lacking. In the last decade, many papers on the use of new urinary and serum biomarkers for the diagnosis and prognostication of AKI have been published. Thus, the question arises which biomarker is a reliable differential diagnostic tool under which circumstances.

METHODS

We searched Medline from inception to April 2012 using medical subject heading and text words for AKI and biomarkers [neutrophil gelatinase-associated lipocalin (NGAL), kidney injury molecule-1 (KIM-1), Cystatin C, interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-18 (IL-18), N-acetyl-glucosaminidase (NAG), glutathione transferases (GST) and liver fatty acid binding protein (LFABP)] to identify relevant papers in five different settings (paediatrics, cardiac surgery, emergency department, critically ill and contrast-induced nephropathy).

RESULTS

We included 87 relevant papers, reporting on 74 studies. Depending upon the setting, 7-27 different definitions of AKI were used. Reported diagnostic performance of the different biomarkers was variable from poor to excellent, and no consistent generalizable conclusions can be drawn on their diagnostic value.

CONCLUSIONS

Early diagnosing of AKI in clinical conditions by using new serum and urinary biomarkers remains cumbersome, especially in those settings where timing and aetiology of AKI are not well defined. Putting too much emphasis on markers that have not convincingly proven reliability might lead to incorrect interpretation of clinical trials. Further research in this field is warranted before biomarkers can be introduced in clinical practice.

Prognostic utility of neutrophil gelatinase-associated lipocalin in predicting mortality and cardiovascular events in patients with ST-segment elevation myocardial infarction treated with primary percutaneous coronary intervention

OBJECTIVES

The aim of this study was to investigate the prognostic role of neutrophil gelatinase-associated lipocalin (NGAL) in a large population of patients with ST-segment elevation myocardial infarction.

BACKGROUND

NGAL is a glycoprotein released by damaged renal tubular cells and is a sensitive maker of both clinical and subclinical acute kidney injury. New data have demonstrated that NGAL is also stored in granules of mature neutrophils, and recent data suggest that NGAL may also be involved in the development of atherosclerosis. NGAL is significantly increased in patients with myocardial infarction compared with patients with stable coronary artery disease and healthy subjects. However, the prognostic value of NGAL has never been studied in patients with myocardial infarction.

METHODS

We included 584 consecutive ST-segment elevation myocardial infarction patients admitted to the heart center of Gentofte University Hospital, Denmark, and treated with primary percutaneous coronary intervention, from September 2006 to December 2008. Blood samples were drawn immediately before primary percutaneous coronary intervention. Plasma NGAL levels were measured using a time-resolved immunofluorometric assay. The endpoints were all-cause mortality (n = 69) and the combined endpoints (n = 116) of major adverse cardiac events (MACE) defined as cardiovascular mortality and admission due to recurrent myocardial infarction or heart failure. The median follow-up time was 23 months (interquartile range, 20 to 24 months).

RESULTS

Patients with high NGAL (>75th percentile) had increased risk of all-cause mortality and MACE compared with patients with low NGAL (log-rank test, p < 0.001). After adjustment for confounding risk factors chosen by backward elimination by Cox regression analysis, high NGAL remained an independent predictor of all-cause mortality and MACE (hazard ratio: 2.00; 95% confidence interval: 1.16 to 3.44; p = 0.01 and hazard ratio: 1.51; 95% confidence interval: 1.00 to 2.30; p = 0.05, respectively).

CONCLUSIONS

High plasma NGAL independently predicts all-cause mortality and MACE in ST-segment elevation myocardial infarction patients treated with primary percutaneous coronary intervention.

Clinical review: Biomarkers of acute kidney injury: where are we now?

The recognition that acute kidney injury (AKI) is a significant independent risk factor for morbidity and mortality has resulted in a substantial number of publications over the past 5 years or more. In no small part these have, to a degree, highlighted the inadequacy of conventional markers of renal insufficiency in the acute setting. Much effort has been invested in the identification of early, specific AKI markers in order to aid early diagnosis of AKI and hopefully improve outcome. The search for a 'biomarker' of AKI has seen early promise replaced by a degree of pessimism due to the lack of a clear candidate molecule and variability of results. We outline the major studies described to date as well as discuss potential reasons for the discrepancies observed and suggest that evolution of the field may result in success with ultimately an improvement in patient outcomes.

Association of postoperative proteinuria with AKI after cardiac surgery among patients at high risk

BACKGROUND AND OBJECTIVES

Preoperative proteinuria is associated with a higher incidence of postoperative AKI. Whether the same is true for postoperative proteinuria is uncertain. This study tested the hypothesis that increased proteinuria after cardiac surgery is associated with an increased risk for AKI.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This prospective cohort study included 1198 adults undergoing cardiac surgery at six hospitals between July 2007 and December 2009. Albuminuria, urine albumin-to-creatinine ratio (ACR), and dipstick proteinuria were measured 0-6 hours after surgery. The primary outcome was AKI, defined as a doubling in serum creatinine or receipt of acute dialysis during the hospital stay. Analyses were adjusted for patient characteristics, including preoperative albuminuria.

RESULTS

Compared with the lowest quintile, the highest quintile of albuminuria and highest grouping of dipstick proteinuria were associated with greatest risk for AKI (adjusted relative risks [RRs], 2.97 [95% confidence interval (CI), 1.20-6.91] and 2.46 [95% CI, 1.16-4.97], respectively). Higher ACR was not associated with AKI risk (highest quintile RR, 1.66 [95% CI, 0.68-3.90]). Of the three proteinuria measures, early postoperative albuminuria improved the prediction of AKI to the greatest degree (clinical model area under the curve, 0.75; 0.81 with albuminuria). Similar improvements with albuminuria were seen for net reclassification index (0.55; P<0.001) and integrated discrimination index (0.036; P<0.001).

CONCLUSIONS

Higher levels of proteinuria after cardiac surgery identify patients at increased risk for AKI during their hospital stay.

Acute kidney injury after surgery for congenital heart disease

BACKGROUND

The RIFLE criteria (risk, injury, failure, loss, and end-stage kidney disease) have been used to assess acute kidney injury (AKI) in various populations of critically ill children. There are limited reports of AKI using RIFLE criteria in large pediatric populations undergoing congenital heart disease surgery.

METHODS

Records of patients 18 years and younger who underwent surgery for congenital heart disease between January 2006 and November 2009 were reviewed. The RIFLE score was determined for each patient postoperatively. Multivariate logistic regression analyses were performed to determine risk factors for AKI and the association with clinical outcomes, with subanalyses of patients 1 month of age or younger.

RESULTS

Data for 458 patients (median age, 7.6 months) were collected and analyzed. Evidence of AKI was demonstrated in 234 patients (51%), the vast majority of whom recovered within 48 hours. Younger age, higher RACHS-1 (risk-adjusted classification for congenital heart surgery) category, and longer cardiopulmonary bypass time were associated with development of AKI. Acute kidney injury was associated with longer duration of ventilation and lengths of intensive care unit and hospital stay. Incidence of AKI in patients 1 month of age or younger was 60.9%, of which more than half required greater than 72 hours to recover. In patients 1 month of age or younger, use of cardiopulmonary bypass, lower preoperative serum creatinine, and higher preoperative blood urea nitrogen were associated with AKI, and AKI was the only factor associated with longer intensive care unit and hospital lengths of stay.

CONCLUSIONS

Incidence of AKI based on RIFLE criteria in patients undergoing congenital heart disease surgery is higher than previously reported. Risk factors include age 1 month or younger and use of cardiopulmonary bypass. Acute kidney injury is associated with longer lengths of stay.

Recent advances in acute kidney injury epidemiology

PURPOSE OF REVIEW

Expanding rates of acute kidney injury (AKI) coupled with increasing awareness of its short-term and long-term sequelae have focused efforts to identify patients at risk for this disease and its complications. This review details the recent attempts to identify novel risk factors for AKI, describes further refinements in the diagnostic and prognostic approach using biological markers of injury, and highlights the features of AKI that independently predict poor long-term outcomes.

RECENT FINDINGS

The presence of proteinuria predicts the development of AKI independently of estimated glomerular filtration rate. Initial results from a large prospective study of AKI biomarkers in cardiac surgery indicate lower agreement with serum creatinine as an AKI standard than observed in early studies. AKI severity and duration are important predictors of chronic kidney disease and long-term mortality. A minority of patients surviving AKI with decreased kidney function is seen by a nephrologist.

SUMMARY

Although the pathophysiologic link is unclear, proteinuria is an easily measurable risk factor for AKI worth considering before anticipated procedures or medication exposures carrying nephrologic risk. Investigation extending beyond agreement with serum creatinine is needed to fully understand the diagnostic and prognostic value of AKI biomarkers. Severity and duration are components of AKI that can help risk-stratify survivors in need of monitoring or nephrology referral.

Urinary neutrophil gelatinase-associated lipocalin as biomarker in the differential diagnosis of impairment of kidney function in cirrhosis

BACKGROUND & AIMS

Impairment of kidney function is common in cirrhosis but differential diagnosis remains a challenge. We aimed at assessing the usefulness of neutrophil gelatinase-associated lipocalin (NGAL), a biomarker of tubular damage, in the differential diagnosis of impairment of kidney function in cirrhosis.

METHODS

Two-hundred and forty-one patients with cirrhosis, 72 without ascites, 85 with ascites, and 84 with impaired kidney function, were studied. Urinary levels of NGAL were measured by ELISA.

RESULTS

Patients with impaired kidney function had higher urinary NGAL levels compared to patients with and without ascites. Patients with urinary tract infection (n=25) had higher uNGAL values than non-infected patients. Patients with acute tubular necrosis (ATN) had uNGAL levels markedly higher (417μg/g creatinine (239-2242) median and IQ range) compared to those of patients with pre-renal azotemia due to volume depletion 30 (20-59), chronic kidney disease (CKD) 82 (34-152), and hepatorenal syndrome (HRS) 76 (43-263) μg/g creatinine (p<0.001 for all). Among HRS patients, the highest values were found in HRS-associated with infections, followed by classical (non-associated with active infections) type-1 and type-2 HRS (391 (72-523), 147 (83-263), and 43 (31-74) μg/g creatinine, respectively; p<0.001). Differences in uNGAL levels between classical type 1 HRS and ATN on the one hand and classical type 1 HRS and CKD and pre-renal azotemia on the other were statistically significant (p<0.05).

CONCLUSIONS

uNGAL levels may be useful in the differential diagnosis of impairment of kidney function in cirrhosis. Urinary tract infections should be ruled out because they may increase uNGAL excretion.

Urine biomarkers predict acute kidney injury in newborns

OBJECTIVE

To identify urine biomarkers predictive of acute kidney injury (AKI) in infants admitted to level 2 and 3 neonatal intensive care units with birth weight >2000 g and 5-minute Apgar score ≤ 7.

STUDY DESIGN

A nested case-control study was performed comparing 8 candidate urine AKI biomarkers in infants with AKI (defined as a rise in serum creatinine of at least 0.3 mg/dL or a serum creatinine elevation ≥ 1.7 mg/dL persisting for 3 days) and 24 infants from the described cohort without AKI. Urine was analyzed for neutrophil gelatinase-associated lipocalin, osteopontin, cystatin C, albumin, β(2) microglobulin, epithelial growth factor, uromodulin (UMOD), and kidney injury molecule 1.

RESULTS

Compared with the infants without AKI, those with AKI had higher levels of urine cystatin C (1123 pg/mL [95% CI, 272-4635 pg/mL] vs 90 pg/mL [95% CI, 39-205 pg/mL]; P < .004; area under the receiver operating characteristic curve [AUC] = 0.82), lower levels of UMOD (11.0 pg/mL [95% CI, 5.7-21.4 pg/mL] vs 26.2 pg/mL [95% CI, 17.4-39.4 pg/mL]; P < .03; AUC = 0.77), and lower levels of epithelial growth factor (6.7 pg/mL [95% CI, 4.0-11.3 pg/mL] vs 17.4 pg/mL [95% CI, 12.7-23.8 pg/mL; P = .003; AUC = 0.82). Although the differences were not statistically significant, levels of urine neutrophil-associated gelatinase lipocalin, kidney injury molecule 1, and osteopontin trended higher in infants with AKI.

CONCLUSION

Urinary biomarkers can predict AKI in neonates admitted to level 2 and 3 neonatal intensive care units.

Fibrinogen excretion in the urine and immunoreactivity in the kidney serves as a translational biomarker for acute kidney injury

Fibrinogen (Fg) is significantly up-regulated in the kidney after acute kidney injury (AKI). We evaluated the performance of Fg as a biomarker for early detection of AKI. In rats and mice with kidney tubular damage induced by ischemia/reperfusion (I/R) or cisplatin administration, respectively; kidney tissue and urinary Fg increased significantly and correlated with histopathological injury, urinary kidney injury molecule-1 (KIM-1) and N-acetyl glucosaminidase (NAG) corresponding to the progression and regression of injury temporally. In a longitudinal follow-up of 31 patients who underwent surgical repair of abdominal aortic aneurysm, urinary Fg increased earlier than SCr in patients who developed postoperative AKI (AUC-ROC = 0.72). Furthermore, in a cohort of patients with biopsy-proven AKI (n = 53), Fg immunoreactivity in the tubules and interstitium increased remarkably and was able to distinguish patients with AKI from those without AKI (n = 59). These results suggest that immunoreactivity of Fg in the kidney, as well as urinary excretion of Fg, serves as a sensitive and early diagnostic translational biomarker for detection of AKI.

Serum cystatin C- versus creatinine-based definitions of acute kidney injury following cardiac surgery: a prospective cohort study

BACKGROUND

The primary aim of this study was to compare the sensitivity and rapidity of acute kidney injury (AKI) detection by cystatin C level relative to creatinine level after cardiac surgery.

STUDY DESIGN

Prospective cohort study.

SETTINGS & PARTICIPANTS

1,150 high-risk adult cardiac surgery patients in the TRIBE-AKI (Translational Research Investigating Biomarker Endpoints for Acute Kidney Injury) Consortium.

PREDICTOR

Changes in serum creatinine and cystatin C levels.

OUTCOME

Postsurgical incidence of AKI.

MEASUREMENTS

Serum creatinine and cystatin C were measured at the preoperative visit and daily on postoperative days 1-5. To allow comparisons between changes in creatinine and cystatin C levels, AKI end points were defined by the relative increases in each marker from baseline (25%, 50%, and 100%) and the incidence of AKI was compared based on each marker. Secondary aims were to compare clinical outcomes among patients defined as having AKI by cystatin C and/or creatinine levels.

RESULTS

Overall, serum creatinine level detected more cases of AKI than cystatin C level: 35% developed a ≥25% increase in serum creatinine level, whereas only 23% had a ≥25% increase in cystatin C level (P < 0.001). Creatinine level also had higher proportions meeting the 50% (14% and 8%; P < 0.001) and 100% (4% and 2%; P = 0.005) thresholds for AKI diagnosis. Clinical outcomes generally were not statistically different for AKI cases detected by creatinine or cystatin C level. However, for each AKI threshold, patients with AKI confirmed by both markers had a significantly higher risk of the combined mortality/dialysis outcome compared with patients with AKI detected by creatinine level alone (P = 0.002).

LIMITATIONS

There were few adverse clinical outcomes, limiting our ability to detect differences in outcomes between subgroups of patients based on their definitions of AKI.

CONCLUSIONS

In this large multicenter study, we found that cystatin C level was less sensitive for AKI detection than creatinine level. However, confirmation by cystatin C level appeared to identify a subset of patients with AKI with a substantially higher risk of adverse outcomes.

Associations of urinary levels of kidney injury molecule 1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL) with kidney function decline in the Multi-Ethnic Study of Atherosclerosis (MESA)

BACKGROUND

Whether elevations in levels of urinary biomarkers of tubular injury (urine neutrophil gelatinase-associated lipocalin [NGAL] and kidney injury molecule 1 [KIM-1]) are associated with future risk of kidney disease has not been investigated.

STUDY DESIGN

1:1 nested case-control study.

SETTING & PARTICIPANTS

686 participants in the Multi-Ethnic Study of Atherosclerosis (MESA).

PREDICTOR

NGAL and KIM-1 were measured at baseline, expressed as log-transformed continuous variables, and categorized into deciles.

OUTCOMES

Kidney function was estimated by cystatin C level using the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation. Incident CKD stage 3 was defined as estimated glomerular filtration rate (eGFR) <60 mL/min/1.73 m(2) and an eGFR decrease >1 mL/min/1.73 m(2) per year, and rapid kidney function decrease was defined as decrease ≥3 mL/min/1.73 m(2) per year.

MEASUREMENTS

Cases were defined as persons with eGFR >60 mL/min/1.73 m(2) who subsequently developed incident CKD stage 3 and/or had rapid kidney function decrease by the MESA year-5 visit. Controls were matched for age, sex, race, diabetes, and baseline eGFR. We adjusted for age, hypertension, and presence of albuminuria (albumin-creatinine ratio ≥30 mg/g).

RESULTS

Of 343 cases, 145 had incident CKD stage 3, 141 had rapid kidney function decrease, and 57 had both. Mean eGFR for controls was 81 ± 10 mL/min/1.73 m(2) at baseline and 80 ± 10 mL/min/1.73 m(2) at follow-up compared with 82 ± 13 and 58 ± 10 mL/min/1.73 m(2) for cases. Each doubling of KIM-1 level (in picograms per milliliter) was associated with an OR of 1.15 (95% CI, 1.02-1.29) for incident CKD stage 3 and/or rapid kidney function decrease. Compared with the lowest 90%, the highest decile of KIM-1 level was associated with an OR of 2.02 (95% CI, 1.15-3.56) for the outcome; these associations were independent of albuminuria. NGAL levels (in nanograms per milliliter) were not associated with incident CKD stage 3 and/or rapid kidney function decrease (OR, 1.04; 95% CI, 0.99-1.10). Results were similar when KIM-1 and NGAL levels were standardized for urine creatinine.

LIMITATIONS

The case-control design limits the ability to account for persons who died or were not available for follow-up.

CONCLUSIONS

Urinary KIM-1 level is associated with future risk of kidney disease independent of albuminuria. Urinary biomarkers of tubular injury are a promising tool for identifying persons at risk of CKD.

Effect of high-dose erythropoietin on graft function after kidney transplantation: a randomized, double-blind clinical trial

BACKGROUND AND OBJECTIVES

Delayed graft function (DGF) is associated with adverse long-term outcomes after deceased-donor kidney (DDK) transplantation. Ischemia-reperfusion injury plays a crucial role in the development of DGF. On the basis of promising animal data, this study evaluated any potential benefits of erythropoietin-alfa (EPO-α) given intra-arterially at the time of reperfusion of renal allograft on the degree of allograft function, as well as tubular cell injury measured by urinary biomarkers in the early post-transplant period.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A prospective, randomized, double-blind, placebo-controlled clinical trial was conducted to evaluate the influence of EPO-α administered intraoperatively on the outcomes of DDK transplantations performed at the study center between March 2007 and July 2009.

RESULTS

Seventy-two patients were randomly assigned to EPO-α (n=36) or placebo (n=36). The incidences of DGF, slow graft function, and immediate graft function did not significantly differ between the treatment and control groups (41.7% versus 47.2%, 25.0% versus 36.1%, and 33.3% versus 16.7%, respectively; P=0.24). The groups had similar levels of urinary biomarkers, including neutrophil gelatinase-associated lipocalin and IL-18 at multiple times points soon after transplantation; urinary output during the first 3 postoperative days; 1-month renal function; and BP readings, hemoglobin, and adverse effects during the first month.

CONCLUSIONS

This study did not show any clinically demonstrable beneficial effects of high-dose EPO-α given intra-arterially during the early reperfusion phase in DDK transplant recipients in terms of reducing the incidence of DGF or improving short-term allograft function.

New metrics for assessing diagnostic potential of candidate biomarkers

New tests should improve the diagnostic performance of available tests. The area under the receiver operator characteristic curve has been the "metric of choice" to quantify new biomarker performance. Two new metrics, the integrated discrimination improvement (IDI) and net reclassification improvement (NRI), have been rapidly adopted to quantify the added value of a biomarker to an existing test. These metrics require the development of risk prediction models that calculate the probability of an event for each individual. This study demonstrates the application of these metrics in 528 critically ill patients with risk models of AKI, sepsis, and 30-day mortality to which the biomarker urinary cystatin C was added. Analogous to the receiver operator characteristic curve, we present a new risk assessment plot for visualizing these metrics. The results showed that the NRI was sensitive to the choice of risk threshold. The risk assessment plot identified that the addition of urinary cystatin C to the model decreased the calculated risk for some who did not have sepsis but increased it for others. The category-free NRI for each outcome indicated that most of those without the event had reduced calculated risk. This was driven by very small changes in calculated risk in the AKI and death models. The IDI reflected those small changes. Of the new metrics, the IDI, reported separately for those with and without the events, best represents the value of a new test. The risk assessment plot identified differences in the models not apparent in any of the metrics.

Klotho as a potential biomarker and therapy for acute kidney injury

Klotho is a single-pass transmembrane protein that is highly expressed in the kidney and is known to act as a coreceptor for fibroblast growth factor 23. The extracellular domain can be produced independently or shed from membrane-bound Klotho and functions as an endocrine substance with multiple functions including antioxidation, modulation of ion transport, suppression of fibrosis, and preservation of stem cells. Emerging evidence has revealed that Klotho deficiency is an early event in acute kidney injury (AKI), and a pathogenic factor that exacerbates acute kidney damage and contributes to long-term consequences. Restoration by exogenous supplementation or stimulation of endogenous Klotho might prevent and ameliorate injury, promote recovery, and suppress fibrosis to mitigate development of chronic kidney disease. Although data are still emerging, in this Perspectives article we discuss why this renal-derived protein is a highly promising candidate as both an early biomarker and therapeutic agent for AKI.

Urinary interleukin-18 and urinary neutrophil gelatinase-associated lipocalin predict acute kidney injury following pulmonary valve replacement prior to serum creatinine

BACKGROUND

It is becoming increasingly recognized that manifestations of congenital heart disease (CHD) extend beyond the cardiovascular system. The factors contributing to renal dysfunction in patients with CHD are multifactorial, with acute kidney injury (AKI) at time of cardiac surgery playing a major role. AKI is often diagnosed based on changes in serum creatinine and estimated glomerular filtration rate (eGFR). Such measurements are often late and imprecise. Recent data indicate that urinary biomarkers interleukin-18 (IL-18) and neutrophil gelatinase-associated lipocalin (NGAL) are earlier markers of AKI. We sought to determine the efficacy of urinary IL-18 and NGAL for detecting early AKI in patients undergoing surgical pulmonary valve replacement (PVR).

METHODS

Twenty patients presenting for surgical PVR with a history of previous repair of a conotruncal anomaly were enrolled. Preoperative clinical data were measured and urine samples and serum creatinine were collected at 6, 12, 24, and 72 hours post bypass. Urine was evaluated for NGAL and IL-18. AKI was determined using the Risk, Injury, Failure, Loss and End Stage Renal Disease (RIFLE) classification system.

RESULTS

Using the RIFLE classification system, seven patients (35%) were found to have AKI defined as a drop in the eGFR or an increase in serum creatinine. All seven patients with AKI had marked increase from preoperative baseline in urine IL-18 (sixfold) and NGAL (26-fold). Using NGAL and IL-18, AKI was detected at 6 hours postoperatively, resulting in AKI being identified 12-36 hours prior to detection by conventional methods. No preoperative predictors for AKI were identified.

CONCLUSION

Both NGAL and IL-18 are early predictive biomarkers of AKI, and both increase in tandem after surgical PVR. Importantly, both rise before an increase in creatinine or a decrease in eGFR is present. Monitoring both biomarkers may allow for earlier detection and subsequent interventions to prevent AKI at time of surgery for CHD.

[NGAL, biomarker of acute kidney injury in 2012]

Neutrophil Gelatinase Associated Lipocalin (NGAL) is one of the most promising biomarkers for acute kidney injury (AKI). Although urinary NGAL is intuitively more appropriate to apprehend renal injury, clinical data have accumulated on the potential interest of NGAL measured indifferently in serum or urine. Diagnostic performance of NGAL greatly varies across studies according to different factors such as the type of patients (pediatric versus adult) and the clinical situations (surgery versus intensive care). Overall, NGAL is presented as a useful tool to diagnose and predict AKI outcome but several issues (the absence of a unique pertinent threshold value, the incomplete analytical validation of its measurement and, its apparent limited clinical added value as compared to traditional AKI markers) remain to be addressed in order to definitely recommend its use in clinical practice.

Marking renal injury: can we move beyond serum creatinine?

Acute kidney injury (AKI) is a prevalent and devastating condition associated with significant morbidity and mortality. Despite marked improvements in clinical care, the outcomes for subjects with AKI have shown limited improvement in the past 50 years. A major factor inhibiting clinical progress in this field has been the inability to accurately predict and diagnose early kidney dysfunction. The current gold standard clinical and biochemical criteria for diagnosis of AKI, Risk Injury Failure Loss End-stage renal disease, and its modification, Acute Kidney Injury Network criteria, rely on urine output and serum creatinine, which are insensitive, nonspecific, and late markers of disease. The recent development of a variety of analytic mass spectrometry-based platforms have enabled separation, characterization, detection, and quantification of proteins (proteomics) and metabolites (metabolomics). These high-throughput platforms have raised hopes of identifying novel protein and metabolite markers, and recent efforts have led to several promising novel markers of AKI. However, substantial challenges remain, including the need to systematically evaluate incremental performance of these markers over and beyond current clinical and biochemical criteria for AKI. We discuss the basic issues surrounding AKI biomarker development, highlight the most promising markers currently under development, and discuss the barriers toward widespread clinical implementation of these markers.

Ongoing clinical trials in AKI

AKI is an important public health issue. AKI is a common hospital complication associated with increased in-hospital and long-term mortality, extensive morbidity (including prolonged hospital length of stay), and an estimated annual cost of at least $10 billion in the United States. At present, no specific therapy has been developed to prevent AKI, hasten recovery of kidney function, or abrogate the deleterious systemic effects of AKI. However, recent progress includes establishing a consensus definition of AKI and discovery of novel biomarkers that may allow early detection of AKI. Furthermore, significant insights into the pathophysiology of AKI and its deleterious systemic effects have been gleaned from animal studies. Urgently needed are large, definitive randomized clinical trials testing interventions to prevent and/or treat AKI. This review summarizes and analyzes current ongoing clinical trials registered with clinicaltrials.gov that address prevention or management of AKI. The purpose of this review is to provide a resource for people interested in potential prophylactic and therapeutic approaches to patient care and investigators hoping to plan and execute the next round of randomized clinical trials. Finally, this review discusses research needs that are not addressed by the current clinical trials portfolio and suggests key areas for future research in AKI.

Combination of two urinary biomarkers predicts acute kidney injury after adult cardiac surgery

BACKGROUND

Urinary L-type fatty acid-binding protein (L-FABP) has not been evaluated for adult post-cardiac surgery acute kidney injury (AKI) to date. This study was undertaken to evaluate a biomarker panel consisting of urinary L-FABP and N-acetyl-β-D-glucosaminidase (NAG), a more established urinary marker of kidney injury, for AKI diagnosis in adult post-cardiac surgery patients.

METHODS

This study prospectively evaluated 77 adult patients who underwent cardiac surgery at 2 general hospitals. Urinary L-FABP and NAG were measured before surgery, at intensive care unit arrival after surgery (0 hours), 4, and 12 hours after arrival. The AKI was diagnosed by the Acute Kidney Injury Network criteria.

RESULTS

Of 77 patients, 28 patients (36.4%) developed AKI after surgery. Urinary L-FABP and NAG were significantly increased. However, receiver operating characteristic (ROC) analysis revealed that the biomarkers' performance was statistically significant but limited for clinical translation (area under the curve of ROC [AUC-ROC] for L-FABP at 4 hours 0.72 and NAG 0.75). Urinary L-FABP showed high sensitivity and NAG detected AKI with high specificity. Therefore, we combined these 2 biomarkers, which revealed that this combination panel can detect AKI with higher accuracy than either biomarker measurement alone (AUC-ROC 0.81). Moreover, this biomarker panel improved AKI risk prediction significantly compared with predictions made using the clinical model alone.

CONCLUSIONS

When urinary L-FABP and NAG are combined, they can detect AKI adequately, even in a heterogeneous population of adult post-cardiac surgery AKI. Combining 2 markers with different sensitivity and specificity presents a reasonable strategy to improve the diagnostic performance of biomarkers.

Some biomarkers of acute kidney injury are increased in pre-renal acute injury

Pre-renal acute kidney injury (AKI) is assumed to represent a physiological response to underperfusion. Its diagnosis is retrospective after a transient rise in plasma creatinine, usually associated with evidence of altered tubular transport, particularly that of sodium. In order to test whether pre-renal AKI is reversible because injury is less severe than that of sustained AKI, we measured urinary biomarkers of injury (cystatin C, neutrophil gelatinase-associated lipocalin (NGAL), γ-glutamyl transpeptidase, IL-18, and kidney injury molecule-1 (KIM-1)) at 0, 12, and 24 h following ICU admission. A total of 529 patients were stratified into groups having no AKI, AKI with recovery by 24 h, recovery by 48 h, or the composite of AKI greater than 48 h or dialysis. Pre-renal AKI was identified in 61 patients as acute injury with recovery within 48 h and a fractional sodium excretion <1%. Biomarker concentrations significantly and progressively increased with the duration of AKI. After restricting the AKI recovery within the 48 h cohort to pre-renal AKI, this increase remained significant. The median concentration of KIM-1, cystatin C, and IL-18 were significantly greater in pre-renal AKI compared with no-AKI, while NGAL and γ-glutamyl transpeptidase concentrations were not significant. The median concentration of at least one biomarker was increased in all but three patients with pre-renal AKI. Thus, the reason why some but not all biomarkers were increased requires further study. The results suggest that pre-renal AKI represents a milder form of injury.

Biomarkers predict progression of acute kidney injury after cardiac surgery

Being able to predict whether AKI will progress could improve monitoring and care, guide patient counseling, and assist with enrollment into trials of AKI treatment. Using samples from the Translational Research Investigating Biomarker Endpoints in AKI study (TRIBE-AKI), we evaluated whether kidney injury biomarkers measured at the time of first clinical diagnosis of early AKI after cardiac surgery can forecast AKI severity. Biomarkers included urinary IL-18, urinary albumin to creatinine ratio (ACR), and urinary and plasma neutrophil gelatinase-associated lipocalin (NGAL); each measurement was on the day of AKI diagnosis in 380 patients who developed at least AKI Network (AKIN) stage 1 AKI. The primary end point (progression of AKI defined by worsening AKIN stage) occurred in 45 (11.8%) patients. Using multivariable logistic regression, we determined the risk of AKI progression. After adjustment for clinical predictors, compared with biomarker values in the lowest two quintiles, the highest quintiles of three biomarkers remained associated with AKI progression: IL-18 (odds ratio=3.0, 95% confidence interval=1.3-7.3), ACR (odds ratio=3.4, 95% confidence interval=1.3-9.1), and plasma NGAL (odds ratio=7.7, 95% confidence interval=2.6-22.5). Each biomarker improved risk classification compared with the clinical model alone, with plasma NGAL performing the best (category-free net reclassification improvement of 0.69, P<0.0001). In conclusion, biomarkers measured on the day of AKI diagnosis improve risk stratification and identify patients at higher risk for progression of AKI and worse patient outcomes.