Objective: Postoperative delirium (POD) is a frequent and serious complication following cardiac surgery and is closely linked to systemic inflammatory activation. The systemic immune-inflammation index (SII), derived from routine complete blood count parameters, integrates neutrophil, lymphocyte, and platelet counts and may reflect the perioperative inflammatory milieu. We aimed to evaluate the association between preoperative SII and POD in patients undergoing cardiac surgery.
Methods: This retrospective cohort study included 1,532 adult patients undergoing cardiac surgery. Preoperative SII was calculated as platelet count × neutrophil count / lymphocyte count. Postoperative delirium was assessed using the Confusion Assessment Method for the Intensive Care Unit (CAM-ICU) from the day of surgery through postoperative day 4. POD was defined as at least one positive CAM-ICU assessment. Multivariable logistic regression analysis was performed to evaluate the independent association between SII and POD after adjustment for clinically relevant covariates. Model discrimination was assessed using the area under the receiver operating characteristic curve (AUC). Quartile-based analyses were conducted to explore dose–response relationships.
Results: POD occurred in 365 patients (23.8%). Patients who developed POD had significantly higher preoperative SII values (median log-SII, 6.5 vs 6.3; p < 0.001). In multivariable analysis, higher preoperative SII remained independently associated with POD (adjusted OR 2.16, 95% CI 1.75–2.66, p < 0.001). Age (adjusted OR 1.02 per year, p = 0.001) and prolonged intubation (adjusted OR 1.52, p = 0.020) were also independent predictors. A graded increase in POD risk was observed across increasing SII quartiles, with the highest quartile demonstrating nearly a threefold increased risk compared with the lowest quartile. The model demonstrated moderate discrimination (AUC 0.654).
Conclusions: Elevated preoperative SII is independently associated with postoperative delirium after cardiac surgery. As an inexpensive and widely available biomarker, SII may provide practical value for preoperative risk stratification and for identifying patients at increased risk for delirium.
