Averaged versus Persistent Reduction in Urine Output to Define Oliguria in Critically Ill Patients, an Observational Study

Background: Oliguria is defined as a urine output (UO) of <0.5 ml/kg/h over six hours. There is no consensus as per whether an average or persistent value should be considered. Methods: We analyzed all adults admitted to our intensive care unit between 2010 and 2020 except those on chronic dialysis or who declined consent. We extracted hourly UO and, across six hours sliding time-windows, assessed for the presence of oliguria according to the average (mean UO below threshold) and persistent method (all measurements below threshold). For both methods, we compared oliguria's incidence and association with 90-day mortality, and acute kidney disease (AKD) at hospital discharge. Results: Among 15,253 patients, the average method identified oliguria more often than the persistent method (73% [95%CI 72.3-73.7] versus 54.3% [53.5-55.1]). It displayed a higher sensitivity for the prediction of 90-day mortality (85% [83.6-86.4] vs 70.3% [68.5 - 72]) and AKD at hospital discharge (85.6% [84.2-87] vs 71.8% [70-73.6]). However, its specificity was lower for both outcomes (29.8% [28.9-30.6] vs 49.4% [48.5-50.3] and 29.8% [29-30.7] vs 49.8% [48.9-50.7]). After adjusting for illness severity, comorbidities, age, admission year, weight, gender, and acute kidney injury (AKI) on admission, the absolute difference in mortality attributable to oliguria at population level was similar with both methods (5%). Similar results were obtained when analyses were restricted to patients without AKI on admission, with documented bodyweight, presence of indwelling catheter throughout stay, who did not receive renal replacement therapy or diuretics. Conclusions: The assessment method of oliguria has major diagnostic and prognostic implications. Its definition should be standardized.