Fractional excretion of urea: A simple tool for the differential diagnosis of acute kidney injury in cirrhosis

Current approaches to determine the cause of acute kidney injury (AKI) in patients with cirrhosis are suboptimal. The aim of this study was to determine the utility of fractional excretion of urea (FEUrea) for the differential diagnosis of AKI in patients with cirrhosis. A retrospective analysis was performed in patients (n=50) with cirrhosis and ascites admitted with AKI. Using adjudicated etiology assessment as the reference standard, receiver operating curves and optimal cutoff, sensitivity (Sn), and specificity (Sp) for the diagnosis of prerenal azotemia (PRA), type 1 hepatorenal syndrome (HRS), and acute tubular necrosis (ATN) were derived. Validation was performed in an independent cohort (n=50) and by bootstrap analysis. The causes of AKI (derivation:validation cohorts) were: PRA 21:21, HRS 18:15, and ATN 11:14. Median FEUrea was statistically different across all etiologies of AKI in the derivation cohort (PRA 30.1 vs. HRS 20.2 vs. ATN 43.6; P<0.001) and validation cohort (PRA 23.1 vs. HRS 13.3 vs. ATN 44.7; P<0.001). The area underneath the curve (cutoff, Sn/Sp) for FEUrea was 0.96 (33.4, 85/100) for ATN versus non-ATN, 0.87 (28.7, 75/83) for HRS versus non-HRS, and 0.81 (21.6, 90/61) for PRA versus HRS. When applied to the validation cohort, Sn/Sp were maintained for ATN versus non-ATN (93/97), HRS versus non-HRS (100/63), and for PRA versus HRS (67/80). After bootstrapping, Sn/Sp for FEUrea in the ATN versus non-ATN, HRS versus non-HRS, and PRA versus HRS was 88/96, 63/97, and 55/87, respectively. Conclusion: FEUrea is a promising tool for the differential diagnosis of AKI in patients with cirrhosis. (Hepatology 2018;68:224-233)