Bilirubin influences the predictive effect of body mass index on hospital mortality in critically ill patients.

Introduction: Body mass index (BMI) can predict mortality in critically ill patients. Moreover, mortality is related to increased bilirubin levels. Thus, herein, we aimed to investigate the effect of bilirubin levels on the usefulness of BMI in predicting mortality in critically ill patients. Methods: Data were extracted from the Medical Information Mart for Intensive Care (MIMIC IV) database. Patients were divided into two groups according to their total bilirubin levels within 24 h. Cox proportional hazard regression models were applied to obtain adjusted hazard ratios and 95 % confidence intervals for the correlation between BMI categories and hospital mortality. The dose-response relationship was flexibly modeled using a restricted cubic spline (RCS) with three knots. Results: Of the 14376 patients included, 3.4 % were underweight, 29.3 % were of normal body weight, 32.2 % were overweight, and 35.1 % were obese. For patients with total bilirubin levels <2 mg/dL, hospital mortality was significantly lower in patients with obesity than in normal body weight patients (p < 0.05). However, the opposite results were observed for patients with total bilirubin levels ≥2 mg/dL. The Cox proportional hazard regression models suggested that the risk of death was lower in patients with overweightness and obesity than in normal body weight patients when the total bilirubin levels were <2 mg/dL, but not in the other case (total bilirubin levels ≥2 mg/dL). RCS analyses showed that, for patients with total bilirubin levels <2 mg/dL, the risk of death gradually decreased with increasing BMI. Conversely, for patients with total bilirubin levels ≥2 mg/dL, this risk did not decrease with increasing BMI until reaching obesity, after which it increased rapidly. Conclusion: BMI predicted the risk of death differently in critically ill patients with different bilirubin levels.