Systemic immune-inflammation index combined with blood urea nitrogen to albumin ratio predicts 28-day mortality in sepsis: a retrospective cohort study

Background Sepsis remains a leading cause of mortality in intensive care units, highlighting an urgent need for simple, early prognostic biomarkers to improve risk stratification. The Systemic Immune-Inflammation Index (SII) and the Blood Urea Nitrogen to Albumin Ratio (BAR) reflect distinct pathophysiological axes—inflammatory-immune-coagulation and renal-metabolic dysfunction, respectively. However, their combined prognostic value in sepsis has not been explored. Methods This single-center retrospective cohort study included older patients with sepsis (median age 82 years) admitted between 2022 and 2024. SII and BAR were calculated from admission laboratory data. The primary outcome was 28-day all-cause mortality. Predictive performance was assessed using ROC analysis and multivariable logistic regression, with internal validation via bootstrapping. A nomogram and a two-dimensional risk heatmap were developed to facilitate clinical application. Results For predicting 28-day mortality, BAR (AUC: 0.721 (95% CI: 0.671–0.771)) outperformed SII (AUC: 0.669, 95% CI: 0.610–0.726). High SII (≥1072) and high BAR (≥8.64) were independent risk factors for mortality (OR: 3.613 and 4.471, respectively). The combined SII-BAR model demonstrated superior discrimination (AUC: 0.750, 95% CI: 0.713–0.822) and excellent calibration (Brier score: 0.110). Decision curve analysis confirmed a positive net benefit across clinically relevant threshold probabilities (10–30%). The nomogram and heatmap provided intuitive, individualized risk estimates. Conclusion SII and BAR are independent and complementary predictors of 28-day mortality in sepsis. Their integration into a simple combined model significantly improves prognostic accuracy by capturing both inflammatory-immune and renal-metabolic dysregulation. These readily available biomarkers, together with the proposed visual tools, may facilitate early risk stratification and support personalized management in sepsis.