The Relationship Between Systematic Inflammatory Index (SII) and Mortality in Patients Admitted to the Non-Interventional Clinical Research Ethics Committee for Evalation

Systemic inflammation is a fundamental component of critical illness and has a strong association with disease severity and mortality in intensive care units. The Systemic Immune-Inflammation Index (SII), calculated from neutrophil, lymphocyte, and platelet counts, has emerged as a quantitative marker of systemic immune-inflammatory status. While SII has been studied in various clinical settings, its prognostic value in reanimation ICU patients remains insufficiently defined.

This study is designed as a single-center, retrospective, non-interventional cohort study conducted in the Reanimation Intensive Care Unit of Atatürk University Research Hospital. Adult patients (≥18 years) admitted to the ICU for infectious or non-infectious critical illnesses will be evaluated through retrospective review of hospital electronic medical records (ENLİL Hospital Information Management System).

Patients will be included if admission laboratory data, including complete blood count and biochemical parameters, are available. Exclusion criteria include pregnancy, hematologic malignancy, active chemotherapy, chronic immunosuppression, pulse steroid therapy, known severe thrombocytopenia (<50×10⁹/L), massive bleeding or massive transfusion within the first 24 hours, and incomplete clinical or laboratory data.

The primary outcome of the study is the association between admission SII levels and ICU mortality. Secondary outcomes include comparison of mortality rates between infectious and non-infectious admission diagnoses, evaluation of correlations between SII and APACHE II and GCS scores, and assessment of whether elevated SII is an independent risk factor for ICU mortality.

Sample size calculation was performed using G*Power software based on a reference population study, assuming a mean SII value of 515±170 in the control group. To detect a 40-unit difference in SII between infectious and non-infectious groups with 80% power and a 95% confidence level, a minimum of 285 patients per group (total ≥570 patients) is required.