In adult cardiac surgery, the predictive value for AKI of neutrophil gelatinase-associated lipocalin (NGAL) appears to have wide variability. The choice of definition of acute kidney injury (AKI) might, at least in part, account for such variability.Methods. In a prospective study of 100 adult cardiac surgery patients, we assessed the value of postoperative plasma NGAL in predicting AKI according to the degree of severity used for its definition.Results. The predictive value of plasma NGAL varied according to the AKI definition used and was higher for more severe AKI (increase in creatinine >50: mean AUC-ROC 0.79 ? 0.01) compared to less severe AKI (>25: mean AUC-ROC 0.65 ? 0.02); P = 0.001. The discriminatory ability of NGAL for AKI also increased with increasing RIFLE classes (AUC-ROC R: 0.72, I: 0.79, F: 0.80) or AKIN stages (AUC-ROC 1: 0.75, 2: 0.78, 3: 0.81); P = 0.015. It was highest for the prediction of renal replacement therapy (AUC-ROC: 0.83).Conclusions. In adult cardiac surgery patients, the predictive value of NGAL increases with grade of AKI. This observation needs to be taken into account when interpreting any future studies of this biomarker.