Urine Output Calculated Using Actual Body Weight May Result in Overestimation of Acute Kidney Injury for Obese Patients

GOAL: The derived hourly urine output (UO) indexed by body weight is one of the major criteria for the diagnosis of acute kidney injury (AKI). However, it is unclear whether actual body weight (ABW) or ideal body weight (IBW) should be used. This study aims to explore whether UO calculation based on ABW might lead to overestimation of AKI. METHOD: AKI patients identified in the Medical Information Mart for Intensive Care III database by different components of the Kidney Disease Improving Global Outcomes guidelines and different definitions of body weight were retrospectively studied. Hospital and 90-day mortality were compared to decide whether different patient groups had the same outcome. RESULTS: In the cohort of 14,725 patients, AKI was identified in 4,298 (29.19%) and 3,060 (20.78%) patients respectively when ABW or IBW was used (P < 0.01). Multivariate logistic regression revealed that AKI patients identified by UO calculated from ABW had similar hospital and 90-day mortality to that of patients with no evidence of AKI. Whereas AKI patients identified by serum creatinine changes, or those identified by both ABW and IBW, had twice higher the risks of hospital death and about 1.5 times higher the risks of 90-day death compared with thoese with no evidence of AKI. Results were confirmed in two separate sensitivity analyses where patients whose admission creatinine levels were within the normal reference ranges and patients identified as sepsis were studied. CONCLUSIONS: Calculating hourly body weight normalized UO using ABW may lead to underestimation of UO and overestimation of AKI.