Blood urea nitrogen as a predictor of 28-day mortality in ICU patients with multiple myeloma: A retrospective cohort study (MIMIC-IV, 2008-2019).

Multiple myeloma patients frequently develop acute kidney injury; yet, the prognostic value of early blood urea nitrogen (BUN) in the critical care setting remains unclear. This retrospective cohort study included 304 critically ill adults with multiple myeloma to evaluate BUN as a readily available biomarker for early risk stratification. Using the Medical Information Mart for Intensive Care-IV 3.1 database (2008-2019), we analyzed first-time intensive care unit admissions. BUN and all covariates were obtained within 24 hours of admission. Cox regression, restricted cubic splines, and Kaplan-Meier analyses estimated the association between BUN and 28-day all-cause mortality. Mean age was 72.6 ± 10.8 years; 61.5% were male. BUN tertiles were T1 <23 mg/dL, T2 23-44 mg/dL, and T3 >44 mg/dL. After multivariable adjustment, each 1 mg/dL rise in BUN increased mortality risk by 1% (hazard ratio [HR] 1.01, 95% confidence interval [CI] 1.01, 1.02). Compared with T1, 28-day mortality risk was higher in T2 (HR 3.47, 95% CI 1.45, 8.30) and T3 (HR 6.96, 95% CI 2.61, 18.61). A threshold effect was observed at 52.57 mg/dL, above which risk plateaued. Survival curves diverged early and remained separated across tertiles (log-rank P < .001); findings were consistent across predefined subgroups. These findings suggest that early BUN measurement can identify high-risk patients who may benefit from intensified monitoring and timely intervention. BUN levels measured within the first 24 hours of intensive care unit admission are independent, dose-dependent predictors of 28-day mortality in critically ill patients with multiple myeloma, supporting the use of BUN for early risk stratification. As an inexpensive and universally available biomarker, BUN holds promise for guiding clinical decision-making and resource allocation in this vulnerable population.