Michael R. Bennett

Urinary cystatin C as an early biomarker of acute kidney injury following adult cardiothoracic surgery

There is a need to develop early biomarkers of acute kidney injury following cardiac surgery, where morbidity and mortality are increased by its presence. Plasma cystatin C (CyC) and plasma and urine Neutrophil Gelatinase Associated Lipocalin (NGAL) have been shown to detect kidney injury earlier than changes in plasma creatinine in critically ill patients. In order to determine the utility of urinary CyC levels as a measure of kidney injury, we prospectively collected plasma and urine from 72 adults undergoing elective cardiac surgery for analysis. Acute kidney injury was defined as a 25% or greater increase in plasma creatinine or renal replacement therapy within the first 72 hours following surgery. Plasma CyC and NGAL were not useful predictors of acute kidney injury within the first 6 hours following surgery. In contrast, both urinary CyC and NGAL were elevated in the 34 patients who later developed acute kidney injury, compared to those with no injury. The urinary NGAL at the time of ICU arrival and the urinary CyC level 6 hours after ICU admission were most useful for predicting acute kidney injury. A composite time point consisting of the maximum urinary CyC achieved in the first 6 hours following surgery outperformed all individual time points. Our study suggests that urinary CyC and NGAL are superior to conventional and novel plasma markers in the early diagnosis of acute kidney injury following adult cardiac surgery.

Urinary Biomarkers in the Clinical Prognosis and Early Detection of Acute Kidney Injury

BACKGROUND AND OBJECTIVES Several novel urinary biomarkers have shown promise in the early detection and diagnostic evaluation of acute kidney injury (AKI). Clinicians have limited tools to determine which patients will progress to more severe forms of AKI at the time of serum creatinine increase. The diagnostic and prognostic utility of novel and traditional AKI biomarkers was evaluated during a prospective study of 123 adults undergoing cardiac surgery. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS Urinary neutrophil gelatinase-associated lipocalin (NGAL), cystatin C (CyC), kidney injury molecule-1 (KIM-1), hepatocyte growth factor (HGF), π-glutathione-S-transferase (π-GST), α-GST, and fractional excretions of sodium and urea were all measured at preoperative baseline, postoperatively, and at the time of the initial clinical diagnosis of AKI. Receiver operator characteristic curves were generated and the areas under the curve (AUCs) were compared. RESULTS Forty-six (37.4%) subjects developed AKI Network stage 1 AKI; 9 (7.3%) of whom progressed to stage 3. Preoperative KIM-1 and α-GST were able to predict the future development of stage 1 and stage 3 AKI. Urine CyC at intensive care unit (ICU) arrival best detected early stage 1 AKI (AUC = 0.70, P < 0.001); the 6-hour ICU NGAL (AUC = 0.88; P < 0.001) best detected early stage 3 AKI. π-GST best predicted the progression to stage 3 AKI at the time of creatinine increase (AUC = 0.86; P = 0.002). CONCLUSION Urinary biomarkers may improve the ability to detect early AKI and determine the clinical prognosis of AKI at the time of diagnosis.

Improved performance of urinary biomarkers of acute kidney injury in the critically ill by stratification for injury duration and baseline renal function

To better understand the diagnostic and predictive performance of urinary biomarkers of kidney injury, we evaluated γ-glutamyltranspeptidase (GGT), alkaline phosphatase (AP), neutrophil-gelatinase-associated lipocalin (NGAL), cystatin C (CysC), kidney injury molecule-1 (KIM-1), and interleukin-18 (IL-18) in a prospective observational study of 529 patients in 2 general intensive care units (ICUs). Comparisons were made using the area under the receiver operator characteristic curve (AUC) for diagnosis or prediction of acute kidney injury (AKI), dialysis, or death, and reassessed after patient stratification by baseline renal function (estimated glomerular filtration rate, eGFR) and time after renal insult. On ICU entry, no biomarker had an AUC above 0.7 in the diagnosis or prediction of AKI. Several biomarkers (NGAL, CysC, and IL-18) predicted dialysis (AUC over 0.7), and all except KIM-1 predicted death at 7 days (AUC between 0.61 and 0.69). Performance was improved by stratification for eGFR or time or both. With eGFR <60 ml/min, CysC and KIM-1 had AUCs of 0.69 and 0.73, respectively, within 6 h of injury, and between 12 and 36 h, CysC (0.88), NGAL (0.85), and IL-18 (0.94) had utility. With eGFR >60 ml/min, GGT (0.73), CysC (0.68), and NGAL (0.68) had the highest AUCs within 6 h of injury, and between 6 and 12 h, all AUCs except AP were between 0.68 and 0.78. Beyond 12 h, NGAL (0.71) and KIM-1 (0.66) performed best. Thus, the duration of injury and baseline renal function should be considered in evaluating biomarker performance to diagnose AKI.

Novel biomarkers early predict the severity of acute kidney injury after cardiac surgery in adults.

BACKGROUND The purpose of this study was to investigate the ability of neutrophil gelatinase-associated lipocalin (NGAL), cystatin C, and their combination in predicting the duration and severity of acute kidney injury (AKI) after cardiac surgery in adults. METHODS Using data from a prospective observational study of 100 adult cardiac surgical patients, we correlated early postoperative concentrations of plasma NGAL and serum cystatin C with the duration (time during which AKI persisted according to the Acute Kidney Injury Network criteria) and severity of AKI (change in serum creatinine) and with length of stay in intensive care. RESULTS We found a mean AKI duration of 67.2 +/- 41.0 hours which was associated with prolonged hospitalization (p < 0.001). NGAL, cystatin C, and their combination on arrival in intensive care correlated with subsequent AKI duration (all p < 0.01) and severity (all p < 0.001). The area under the receiver operating characteristic curve for AKI prediction was 0.77 (95% confidence interval: 0.63 to 0.91) for NGAL and 0.76 (95% confidence interval: 0.61 to 0.91) for cystatin C on arrival in intensive care. Both markers also correlated with length of stay in intensive care (p = 0.037; p = 0.001). Neutrophil gelatinase-associated lipocalin and cystatin C were independent predictors of AKI duration and severity and of length of stay in intensive care (all p < 0.05). The value of cystatin C on arrival in intensive care appeared to be due to a carry-over effect from preoperative values. CONCLUSIONS Immediately postoperatively, NGAL and cystatin C correlated with and were independent predictors of duration and severity of AKI and duration of intensive care stay after adult cardiac surgery. The combination of both renal biomarkers did not add predictive value.

Neutrophil Gelatinase-Associated Lipocalin Concentrations Predict Development of Acute Kidney Injury in Neonates and Children after Cardiopulmonary Bypass

OBJECTIVES To investigate neutrophil gelatinase-associated lipocalin (NGAL) as an early acute kidney injury (AKI) biomarker after neonatal and pediatric cardiopulmonary bypass (CPB). STUDY DESIGN Serum and urine samples were obtained before and at intervals after CPB from 374 patients. AKI was defined as a serum creatinine (S(Cr)) concentration increase from baseline ≥0.3 mg/dL in neonates and ≥50% in children within 48 hours of CPB. Logistic regression was used to assess predictors and clinical outcomes associated with AKI. RESULTS AKI developed in 30% of patients. Plasma and urine NGAL thresholds significantly increased in patients with AKI at 2 hours after CPB and remained elevated at all points, with 2-hour NGAL the earliest, strongest predictor of AKI. In non-neonates, 2-hour plasma and urine NGAL thresholds strongly correlated with length of hospital stay and severity and duration of AKI. CONCLUSION Plasma and urine NGAL thresholds are early predictive biomarkers for AKI and its clinical outcomes after CPB. In neonates, we recommend a 2-hour plasma NGAL threshold of 100 ng/mL and 2-hour urine NGAL threshold of 185 ng/mL for diagnosis of AKI. In non-neonates, recommended AKI thresholds are 50 ng/mL for both 2-hour plasma and urine NGAL.

Association of noninvasively measured renal protein biomarkers with histologic features of lupus nephritis

OBJECTIVE To investigate the relationship of urinary biomarkers and established measures of renal function to histologic findings in lupus nephritis (LN), and to test whether certain combinations of the above-mentioned laboratory measures are diagnostic for specific histologic features of LN. METHODS Urine samples from 76 patients were collected within 2 months of kidney biopsy and assayed for the urinary biomarkers lipocalin-like prostaglandin D synthase (L-PGDS), α(1) -acid glycoprotein (AAG), transferrin (TF), ceruloplasmin (CP), neutrophil gelatinase-associated lipocalin (NGAL), and monocyte chemotactic protein 1 (MCP-1). Using nonparametric analyses, levels of urinary biomarkers and established markers of renal function were compared with histologic features seen in LN, i.e., mesangial expansion, capillary proliferation, crescent formation, necrosis, wire loops, fibrosis, tubular atrophy, and epimembranous deposits. The area under the receiver operating characteristic curve (AUC) was calculated to predict LN activity, chronicity, or membranous LN. RESULTS There was a differential increase in levels of urinary biomarkers that formed a pattern reflective of specific histologic features seen in active LN. The combination of MCP-1, AAG, and CP levels plus protein:creatinine ratio was excellent in predicting LN activity (AUC 0.85). NGAL together with creatinine clearance plus MCP-1 was an excellent diagnostic test for LN chronicity (AUC 0.83), and the combination of MCP-1, AAG, TF, and creatinine clearance plus C4 was a good diagnostic test for membranous LN (AUC 0.75). CONCLUSION Specific urinary biomarkers are associated with specific tissue changes observed in conjunction with LN activity and chronicity. Especially in combination with select established markers of renal function, urinary biomarkers are well-suited for use in noninvasive measurement of LN activity, LN chronicity, and the presence of membranous LN.

Serum Cystatin C Is an Early Predictive Biomarker of Acute Kidney Injury after Pediatric Cardiopulmonary Bypass

BACKGROUND AND OBJECTIVES Acute kidney injury (AKI) is a frequent complication of cardiopulmonary bypass (CPB). Serum creatinine (SCr), the current standard, is an inadequate marker for AKI since a delay occurs before SCr rises. Biomarkers that are sensitive and rapidly measurable could allow early intervention and improve patient outcomes. We investigated the value of serum cystatin C as an early biomarker for AKI after pediatric CPB. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS We analyzed data from 374 prospectively enrolled children undergoing CPB. Serum samples were obtained before and at 2, 12, and 24 hours after CPB. Cystatin C was quantified by nephelometry. The primary outcome was AKI, defined as a > or =50% increase in SCr. Secondary outcomes included severity and duration of AKI, hospital length of stay, and mortality. A multivariable stepwise logistic regression analysis was used to assess predictors of AKI. RESULTS One hundred nineteen patients (32%) developed AKI using SCr criteria. Serum cystatin C concentrations were significantly increased in AKI patients at 12 hours after CPB (P < 0.0001) and remained elevated at 24 hours (P < 0.0001). Maximal sensitivity and specificity for prediction of AKI occurred at a 12-hour cystatin C cut-off of 1.16 mg/L. The 12-hour cystatin C strongly correlated with severity and duration of AKI as well as length of hospital stay. In multivariable analysis, 12-hour cystatin C remained a powerful independent predictor of AKI. CONCLUSION Serum cystatin C is an early predictive biomarker for AKI and its clinical outcomes after pediatric CPB.

Tolerance of the Human Kidney to Isolated Controlled Ischemia

Tolerance of the human kidney to ischemia is controversial. Here, we prospectively studied the renal response to clamp ischemia and reperfusion in humans, including changes in putative biomarkers of AKI. We performed renal biopsies before, during, and after surgically induced renal clamp ischemia in 40 patients undergoing partial nephrectomy. Ischemia duration was >30 minutes in 82.5% of patients. There was a mild, transient increase in serum creatinine, but serum cystatin C remained stable. Renal functional changes did not correlate with ischemia duration. Renal structural changes were much less severe than observed in animal models that used similar durations of ischemia. Other biomarkers were only mildly elevated and did not correlate with renal function or ischemia duration. In summary, these data suggest that human kidneys can safely tolerate 30-60 minutes of controlled clamp ischemia with only mild structural changes and no acute functional loss.

Early postoperative serum cystatin C predicts severe acute kidney injury following pediatric cardiac surgery

In this multicenter, prospective study of 288 children (half under 2 years of age) undergoing cardiac surgery, we evaluated whether the measurement of pre- and postoperative serum cystatin C (CysC) improves the prediction of acute kidney injury (AKI) over that obtained by serum creatinine (SCr). Higher preoperative SCr-based estimated glomerular filtration rates predicted higher risk of the postoperative primary outcomes of stage 1 and 2 AKI (adjusted odds ratios (ORs) 1.5 and 1.9, respectively). Preoperative CysC was not associated with AKI. The highest quintile of postoperative (within 6 h) CysC predicted stage 1 and 2 AKI (adjusted ORs of 6 and 17.2, respectively). The highest tertile of percent change in CysC independently predicted AKI, whereas the highest tertile of SCr predicted stage 1 but not stage 2 AKI. Postoperative CysC levels independently predicted longer duration of ventilation and intensive care unit length of stay, whereas the postoperative SCr change only predicted longer intensive care unit stay. Thus, postoperative serum CysC is useful to risk-stratify patients for AKI treatment trials. More research, however, is needed to understand the relation between preoperative renal function and the risk of AKI.

Furosemide Stress Test and Biomarkers for the Prediction of AKI Severity

Clinicians have access to limited tools that predict which patients with early AKI will progress to more severe stages. In early AKI, urine output after a furosemide stress test (FST), which involves intravenous administration of furosemide (1.0 or 1.5 mg/kg), can predict the development of stage 3 AKI. We measured several AKI biomarkers in our previously published cohort of 77 patients with early AKI who received an FST and evaluated the ability of FST urine output and biomarkers to predict the development of stage 3 AKI (n=25 [32.5%]), receipt of RRT (n=11 [14.2%]), or inpatient mortality (n=16 [20.7%]). With an area under the curve (AUC)±SEM of 0.87±0.09 (P<0.0001), 2-hour urine output after FST was significantly better than each urinary biomarker tested in predicting progression to stage 3 (P<0.05). FST urine output was the only biomarker to significantly predict RRT (0.86±0.08; P=0.001). Regardless of the end point, combining FST urine output with individual biomarkers using logistic regression did not significantly improve risk stratification (ΔAUC, P>0.10 for all). When FST urine output was assessed in patients with increased biomarker levels, the AUC for progression to stage 3 improved to 0.90±0.06 and the AUC for receipt of RRT improved to 0.91±0.08. Overall, in the setting of early AKI, FST urine output outperformed biochemical biomarkers for prediction of progressive AKI, need for RRT, and inpatient mortality. Using a FST in patients with increased biomarker levels improves risk stratification, although further research is needed.