Neutrophil gelatinase-associated lipocalin and acute kidney injury after cardiac surgery: the effect of baseline renal function on diagnostic performance

Abstract

Background and objectives: Neutrophil gelatinase-associated lipocalin (NGAL) is rapidly released by renal tubules after injury, potentially allowing early identification of acute kidney injury (AKI) after cardiac surgery. However, the diagnostic performance of NGAL has varied widely in clinical studies, and it remains unknown what factors modify the relationship between NGAL and AKI. We hypothesized the relationship between urinary NGAL and AKI would vary with baseline renal function, allowing a stratified analysis to improve diagnostic performance of this novel biomarker.

Design, setting, participants, & measurements: We performed a prospective observational study in 426 adult cardiac surgical patients. Urinary NGAL was serially determined, commencing preoperatively and continuing 24 hours postoperatively. AKI was defined as increase in serum creatinine from baseline by either >50% or >0.3 mg/dl within 48 hours postoperatively. Patients were stratified by baseline estimated GFR (eGFR). NGAL levels were compared between patients with and without AKI and diagnostic characteristics determined according to baseline eGFR.

Results: In patients with baseline eGFR >or=60 ml/min, urinary NGAL was higher at all postoperative time points in patients who developed AKI compared with those who did not. In patients with baseline eGFR <60 ml/min, urinary NGAL did not differ at any time between those who did and those who did not develop AKI. Postoperative NGAL best identified AKI in patients with baseline eGFR 90 to 120 ml/min.

Conclusions: The relationship between urinary NGAL and AKI after cardiac surgery varies with baseline renal function, with optimal discriminatory performance in patients with normal preoperative function.

Figure 1.

Urinary NGAL (ng/ml) at serial time points after CPB, comparing patients who did or did not develop AKI. For each comparison, patients are stratified according to preoperative eGFR above or below 60 ml/min. Shaded boxes represent median and interquartile range for NGAL levels. Comparisons are made between patients with and without AKI, within eGFR strata, at each time point using the Mann-Whitney U test with P values annotated.

Figure 2.

ROC curve for urinary NGAL at 24 hours postoperatively, stratified by baseline eGFR (ml/min). The AUC is significantly greater for patients with preoperative eGFR 90 to 120 ml/min compared with all other groups (P < 0.003).