Prognostic Value of Systemic Immune-Inflammation Index (SIII) in Predicting Major Adverse Cardiac Events (MACE) in Patients Undergoing Percutaneous Coronary Intervention (PCI): An Observational Study

Abstract Background Major adverse cardiovascular events (MACE) continue after percutaneous coronary intervention (PCI) despite advances in revascularisation. Systemic inflammation contributes to atherosclerosis and plaque instability. The Systemic Immune-Inflammation Index (SII) is an inexpensive, easily calculated biomarker derived from routine blood counts that integrates inflammatory and immune pathways. This study evaluated its prognostic value for MACE after PCI. Methods This single-center retrospective observational study included 400 consecutive adults undergoing PCI for stable angina or acute coronary syndromes over 1 year. Pre-procedural complete blood counts were used to calculate SII [(platelet × neutrophil)/lymphocyte]. The primary outcome was in-hospital MACE, comprising all-cause mortality, non-fatal myocardial infarction, and ischemic stroke. Patients were stratified by SII quartiles. ROC analysis and multivariable logistic regression assessed predictive performance and independent associations. Results MACE occurred in 6.8% (n = 27). SII was significantly higher in patients with MACE than without (1127.19 ± 335.99 vs. 599.92 ± 324.55 ×10⁹/L; p < 0.001). High SYNTAX scores (> 27) occurred in 72% of the highest versus 1% of the lowest SII quartile (p < 0.001). Increasing SII was associated with higher troponin positivity and lower discharge left ventricular ejection fraction (p < 0.01). ROC analysis showed excellent predictive ability (AUC 0.886; 95% CI 0.827–0.945), with an optimal cut-off ≥ 776 ×10⁹/L. Multivariable analysis identified SII (adjusted OR 5.36 per quartile increase; p = 0.001) and diabetes mellitus (adjusted OR 4.41; p = 0.024) as independent predictors of MACE. Conclusion Elevated SII independently predicts MACE after PCI and may serve as a simple, cost-effective risk-stratification tool in routine clinical practice.