Systemic Immune-Inflammation Index (SII) as a Predictor of Short-Term Mortality Risk in Sepsis-Associated Acute Kidney Injury: A Retrospective Cohort Study

BACKGROUND Sepsis-associated acute kidney injury (SA-AKI) is linked to high mortality rates and an unfavorable prognosis. Early identification of patients with poor prognosis is crucial. This study aimed to investigate the relationship between the systemic immune-inflammation index (SII) and mortality in this specific patient population. MATERIAL AND METHODS This retrospective cohort study used data from the Medical Information Mart for Intensive Care IV database. Data on patient demographics, comorbidities, vital signs, laboratory parameters, treatment usage, acute kidney injury staging, and renal replacement therapy were collected within 48 h of intensive care unit admission. Restricted cubic splines, Kaplan-Meier curves, and Cox regression models were used for analysis. Stratified analyses were performed on the basis of various factors. RESULTS In total, 7856 patients were included, with a median age of 66.9 years and a male-to-female ratio of 57.7%-42.3%. A J-shaped relationship was observed between SII and mortality risk. The lowest mortality risk occurred at an SII of 760.078×10⁹/L. Compared to the reference group (second quartile of SII), the highest and third quartiles had increased 28-day mortality risk, with adjusted hazard ratios (HRs) of 1.33 (1.16-1.52) and 1.55 (1.36-1.77), respectively. Although a trend towards higher mortality hazard was observed in the lowest SII group (Q1), it was not statistically significant, with an adjusted HR of 1.15 (1-1.32). CONCLUSIONS In patients with SA-AKI, both low and high SII were associated with increased short-term mortality risk. The lowest mortality risk was observed at an SII of 760.078×10⁹/L within a 28-day period.