The nature and discriminatory value of urinary neutrophil gelatinase-associated lipocalin in critically ill patients at risk of acute kidney injury.

BACKGROUND: Different molecular forms of urinary neutrophil gelatinase-associated lipocalin (NGAL) have recently been discovered. We aimed to explore the nature, source and discriminatory value of urinary NGAL in intensive care unit (ICU) patients.

METHODS: We simultaneously measured plasma NGAL (pNGAL), urinary NGAL (uNGAL), and estimated monomeric and homodimeric uNGAL contribution using Western blotting-validated enzyme-linked immunosorbent assays [uNGAL(E1) and uNGAL(E2)] and their calculated ratio in 102 patients with the systemic inflammatory response syndrome and oliguria, and/or a creatinine rise of >25 μmol/L.

MEASUREMENTS AND MAIN RESULTS: Bland-Altman analysis demonstrated that, despite correlating well (r = 0.988), uNGAL and uNGAL(E1) were clinically distinct, lacking both accuracy and precision (bias: 266.23; 95% CI 82.03-450.44 ng/mg creatinine; limits of agreement: -1,573.86 to 2,106.32 ng/mg creatinine). At best, urinary forms of NGAL are fair (area under the receiver operating characteristic [AUROC] ≤0.799) predictors of renal or patient outcome; most perform significantly worse. The 44 patients with a primarily monomeric source of uNGAL had higher pNGAL (118.5 ng/ml vs. 72.5 ng/ml; p < 0.001), remaining significant following Bonferroni correction.

CONCLUSIONS: uNGAL is not a useful predictor of outcome in this ICU population. uNGAL patterns may predict distinct clinical phenotypes. The nature and source of uNGAL are complex and challenge the utility of NGAL as a uniform biomarker.