Preoperative Systemic Immune-Inflammation Index and Clinical Outcomes After Coronary Artery Bypass Grafting: A Systematic Review and Meta-analysis

Background: The systemic immune-inflammation index (SII) is a routinely available blood-derived marker with potential prognostic relevance after coronary artery bypass grafting (CABG). We evaluated associations between preoperative SII and postoperative outcomes. Methods: PubMed, Embase, Scopus and Web of Science were searched to 1 October 2026, with supplementary searches through 14 October 2026. Peer-reviewed observational studies evaluating preoperative SII in adults undergoing CABG were eligible. Methodological quality was assessed using the Newcastle-Ottawa Scale. Compatible categorical associations and exploratory adjusted continuous mortality effects were pooled separately using restricted maximum-likelihood random-effects models; other findings were summarized descriptively. Results: Thirty-one reports were included, with an aggregate sample of 30,976 and possible cohort overlap; five contributed to meta-analysis. Two off-pump cohorts comprising 1,378 patients and 180 atrial-fibrillation events yielded an imprecise crude association with postoperative atrial fibrillation (OR 1.60, 95%CI 0.53-4.82; I2=68%). An exploratory crude synthesis indicated greater intra-aortic balloon pump use with higher SII (OR 2.45, 95%CI 1.16-5.18; I2=51%). Two cohorts with differing follow-up yielded an exploratory adjusted mortality HR of 1.11 (95% Wald CI 1.05-1.18) per study-specific SD increase in log-SII. In one 624-patient cohort, high SII was associated with postoperative acute kidney injury after adjustment (OR 3.35, 95% CI 2.25-4.97).Apparent atrial-fibrillation discrimination varied (AUC 0.587-0.946). Conclusions: Preoperative SII showed associations with selected adverse outcomes after CABG, supporting further evaluation as an accessible prognostic marker. Its incremental value beyond established clinical risk models requires validation. Keywords: Systemic immune-inflammation index; Coronary artery bypass grafting; Atrial fibrillation; Prognosis; Systematic review