Systemic Immune-Inflammation Index (SII) is a hemogram-derived biomarker reflecting the balance between systemic inflammation and immune response. In critically ill patients, inflammatory burden at admission may play a key role in prognosis. This retrospective cohort study aims to evaluate the association between admission SII levels and intensive care unit (ICU) mortality in adult patients admitted to a reanimation ICU. Secondary objectives include comparison of mortality according to infectious versus non-infectious admission diagnoses and assessment of the relationship between SII and established severity scores, including APACHE II and Glasgow Coma Scale (GCS). The findings may support the use of SII as a rapid, inexpensive prognostic marker in critically ill patients.

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.

Statistical analysis will be conducted using appropriate correlation tests (Pearson or Spearman) based on data distribution. As a retrospective, non-interventional study, no additional procedures will be performed, and patient confidentiality will be strictly maintained. The study involves no external funding and no declared conflicts of interest.

Inclusion Criteria:

• Age 18 years or older
• Admission to the reanimation intensive care unit
• Availability of admission complete blood count and biochemical laboratory data
• Admission due to infectious or non-infectious critical illness

Exclusion Criteria:

• Pregnancy
• Hematologic malignancy
• Active chemotherapy
• Chronic immunosuppression
• Pulse steroid therapy
• Known thrombocytopenia (platelet count <50×10^9/L)
• Massive bleeding or massive transfusion within the first 24 hours of ICU admission
• Incomplete clinical or laboratory data

Adult patients aged 18 years or older admitted to the reanimation intensive care unit for infectious or non-infectious critical illness, with available admission laboratory data