Defining Reduced Urine Output in Neonatal ICU: Importance for Mortality and Acute Kidney Injury Classification

Background: Acute kidney injury (AKI) is an independent risk factor for mortality in adults and children. Generally, urine output (UO) < 1 mL/kg/h is accepted as oliguria in neonates, although it has not been systematically studied. pRIFLE criteria suggest UO cut-offs similar to those of the adult population (0.3 and 0.5 mL/kg/h). The aim of the present study was to investigate UO in correlation with mortality in critically ill neonates and suggest changes in the pRIFLE definition of reduced diuresis.

Methods: A retrospective cohort study was performed in an eight-bed neonatal intensive care unit (NICU). UO was systematically measured by diaper weight each 3 h. Discriminatory capacity to predict mortality of UO was measured and patients were divided according to UO ranges: G1 > 1.5 mL/kg/h; G2 1.0-1.5 mL/kg/h; G3 0.7-1.0 mL/kg/h and G4 < 0.7 mL/kg/h. These ranges were incorporated to pRIFLEGFR criteria and its performance was evaluated.

Results: Of 384 patients admitted at the NICU during the study period, 72 were excluded and overall mortality was 12.8%. UO showed good performance for mortality prediction (area under the curve 0.789, P < 0.001). There was a stepwise increase in hospital mortality according to UO groups after controlling for SNAPPE-II and diuretic use. Using these UO ranges with pRIFLE improves its discriminatory capacity (area under the receiver operating characteristic curve 0.882 versus 0.693, P < 0.05).

Conclusions: UO is a predictor of mortality in NICU. An association between a UO threshold < 1.5 mL/kg/h and mortality was observed, which is higher than the previously published pRIFLE thresholds. Adopting higher values of UO in pRIFLE criteria can improve its capacity to detect AKI severity in neonates.