Averaged versus Persistent Reduction in Urine Output to Define Oliguria in Critically Ill Patients

Key Points When assessing urine output, consideration of an average or persistent value below a threshold has important diagnostic and prognostic implications Seventy-three percent (95% confidence interval, 72.3 to 73.7) of patients had oliguria by the average method versus 54.3% (53.5 to 55.1) by the persistent method. Background Oliguria is defined as a urine output (UO) of <0.5 ml/kg per hour over 6 hours. There is no consensus as per whether an average or persistent value should be considered. Methods We analyzed all adults admitted to a tertiary intensive care unit between 2010 and 2020, except those on chronic dialysis or who declined consent. We extracted hourly UO and, across 6-hour sliding time windows, assessed for the presence of oliguria according to the average (mean UO below threshold) and persistent (all measurements below a threshold) methods. For both methods, we compared oliguria's incidence and association with 90-day mortality and acute kidney disease at hospital discharge. Results Among 15,253 patients, the average method identified oliguria more often than the persistent method (73% [95% confidence interval, 72.3 to 73.7] versus 54.3% [53.5 to 55.1]). It displayed a higher sensitivity for the prediction of 90-day mortality (85% [83.6 to 86.4] versus 70.3% [68.5 to 72]) and acute kidney disease at hospital discharge (85.6% [84.2 to 87] versus 71.8% [70 to 73.6]). However, its specificity was lower for both outcomes (29.8% [28.9 to 30.6] versus 49.4% [48.5 to 50.3] and 29.8% [29 to 30.7] versus 49.8% [48.9 to 50.7]). After adjusting for illness severity, comorbidities, age, admission year, weight, sex, and AKI on admission, the absolute difference in mortality attributable to oliguria at the population level was similar with both methods (5%). Similar results were obtained when analyses were restricted to patients without AKI on admission, with documented body weight, with presence of indwelling catheter throughout stay, and who did not receive KRT or diuretics. Conclusions The assessment method of oliguria has major diagnostic and prognostic implications. Its definition should be standardized.