Photo Credit: EvgeniyShkolenko

The following is a summary of “Comparison of methods to normalize urine output in critically ill patients: a multicenter cohort study,” published in the December 2024 issue of Critical Care by Monard et al. 

Oliguria diagnosis involves normalizing urine output (UO) by body weight, the rationale and the appropriate method for this normalization remain unclear.  

Researchers conducted a retrospective study to examine how the method used to normalize UO affected oliguria incidence and its association with outcomes.  

They included all adult individuals admitted to a Swiss ICU (derivation cohort) and the US MIMIC-IV database (validation cohort), excluding those on hemodialysis, those who declined consent, or those with fewer than 6 consecutive UO measurements and variables such as ideal body weight, body mass index, body surface area, and adjusted body weight were assessed to identify the best predictor for UO, most closely linked to mean UO during ICU stay. Oliguria incidence and its link to 90-day mortality and acute kidney disease (AKD) at discharge were compared based on normalization by actual body weight (ABW) or the best UO predictor.  

The results showed the derivation and validation cohorts included 15,322 and 28,610 individuals, respectively. Patients in the validation cohort were heavier (mean ABW 81 vs 75 kg), older (65 vs 62 years), and had a lower Simplified Acute Physiology Score II (SAPS-II) score (38 vs 43). Ideal body weight (IBW) was identified as the best predictor for UO. Oliguria incidence increased with ABW normalization but remained stable with IBW normalization. Using IBW improved the association between oliguria and 90-day mortality and AKD, increasing correct classification from 37.6% to 48.3% for mortality and 37.8% to 47% for AKD. These results remained consistent after adjusting for sex and SAPS-II score and were confirmed in sensitivity analyses.  

Investigators concluded the normalizing UO by IBW resulted in a stable incidence of oliguria across weight categories and enhanced the association between oliguria and outcomes, making IBW the preferred method for patients with critical illness. 

Source: ccforum.biomedcentral.com/articles/10.1186/s13054-024-05200-x