Prognostic impact of systemic immune-inflammation index (SII) on infarct size and clinical outcomes in patients with ST-segment elevation myocardial infarction.

Background: Systemic immune-inflammation index (SII), calculated as platelet count × neutrophil count/lymphocyte count, is a novel and easily accessible inflammatory marker. Its prognostic value in predicting infarct size and major adverse cardiovascular events (MACE) after percutaneous coronary intervention (PCI) in patients with ST-segment elevation myocardial infarction (STEMI) remains to be fully explored. Methods: We analyzed 421 patients who underwent primary percutaneous coronary intervention (PCI) within 12 h of symptom onset, enrolled in a prospective multicenter registry (NCT03768453).All patients received immediate admission blood tests for SII calculation (platelet × neutrophil/lymphocyte counts) and completed standardized CMR imaging within 10 days post-PCI.Receiver operating characteristic (ROC) analysis identified the optimal SII cut-off value (914) to predict large infarct size (≥20 % of left ventricular mass). Patients were stratified into high (≥914) and low (<914) SII groups. The relationships between SII, infarct size, and MACE were analyzed using multivariate logistic and Cox regression models. Results: Patients with high SII had significantly larger infarct size (median 29.0 % vs. 22.3 %, p < 0.001). SII ≥ 914 was independently associated with large infarct size (OR 1.889, 95 %CI: 1.100-3.242, p = 0.021) and higher incidence of MACE (HR 1.874, 95 % CI: 1.255-2.796, p = 0.002). Conclusions: Elevated SII (≥914) independently associates with larger infarct size and increased MACE risk post-PCI, suggesting potential utility in risk stratification.