Baseline Interleukin-6 in Sepsis: Mortality Risk Stratification and Survival Analysis in a Prospective Cohort

Background/Objectives: Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Identifying reliable biomarkers that reflect the underlying immune pathophysiology of sepsis and support early risk stratification remains a major clinical priority. This prospective study aimed to evaluate the prognostic value of interleukin-6 (IL-6) measured at ICU admission in patients with sepsis and septic shock. Methods: This prospective observational study included adult patients with sepsis and septic shock admitted to the Intensive Care Unit (ICU) of the Clinical Hospital of Infectious Diseases Constanța between 2021 and 2025. Receiver operating characteristic (ROC) curve analysis with DeLong comparisons, Kaplan–Meier survival analysis, and Cox proportional hazards regression modeling were performed to assess the association between baseline IL-6 levels, in-hospital mortality, and time to death. Results: Among the analyzed biomarkers, IL-6 demonstrated the highest discriminatory performance for in-hospital mortality (AUC = 0.956; 95% CI: 0.893–0.987; p < 0.0001). The optimal cut-off value (>135.14 pg/mL) yielded a sensitivity of 87.65% and specificity of 92.86% (Youden index = 0.805). However, despite this excellent discrimination between survivors and non-survivors, baseline IL-6 levels were not significantly associated with time-to-death in Cox proportional hazards regression analysis. Conclusions: Admission IL-6 showed excellent discriminatory performance for mortality risk stratification but was not associated with survival duration in time-to-event analyses. These findings suggest that IL-6 should be interpreted primarily as an early risk stratification biomarker rather than a predictor of survival duration in patients with sepsis.