Impact of in-hospital body mass index variation on 28-day mortality in critically ill surgical patients: a multi-center retrospective analysis

Background The prognostic value of dynamic body mass index (BMI) changes during hospitalization in surgical intensive care unit (ICU) patients admitted emergently remains unclear. This study aimed to investigate the association between in-hospital BMI change and 28-day mortality in this high-risk population. Methods This retrospective cohort study utilized data from the eICU Collaborative Research Database (2014–2015). A total of 20,543 adult surgical ICU patients admitted via the emergency department (ED) were included. BMI change was calculated as discharge BMI minus admission BMI. Multivariable Cox regression, restricted cubic splines, and subgroup analyses were employed to evaluate the association between BMI change and mortality. Results The 28-day ICU mortality was 4.70%. BMI change exhibited a U-shaped, non-linear association with death: risk declined modestly as BMI rose toward the nadir of −1.75 kg/m 2 , then increased sharply thereafter. Each additional kg/m 2 above this threshold raised mortality by 9% (HR 1.09, 95% CI 1.05–1.12, p < 0.0001). Patients in the highest BMI-gain quartile faced a 52% higher risk than those in the lowest quartile (HR 1.52, 95% CI 1.27–1.82, p < 0.0001). Dynamic BMI change outperformed static BMI or weight measures (AUC 57.9). Conclusion In-hospital BMI change is a significant predictor of 28-day mortality in surgical ICU patients admitted via the ED. A moderate reduction in BMI (−1.75 kg/m 2 ) was associated with the lowest mortality risk. Dynamic BMI monitoring may enhance risk stratification and guide personalized fluid management in this population.