Prognostic implications of the Systemic Immune-Inflammation (SII) index in patients with chronic coronary syndrome with and without diabetes undergoing PCI

The systemic immune-inflammation index (SII: platelet×neutrophil÷lymphocyte) is a robust marker of systemic inflammatory status. Inflammation is widely acknowledged to play a central role in the pathogenesis and prognosis of repeat revascularization(RR) and in-stent restenosis (ISR) after percutaneous coronary intervention (PCI). Recent evidence suggests that elevated SII independently predicts adverse outcomes in patients with acute myocardial infarction, particularly among those with diabetes mellitus (DM). This study aimed to evaluate the prognostic value of SII for predicting RR and ISR in diabetic patients with chronic coronary syndrome (CCS) undergoing PCI. A total of 2,079 patients were enrolled, including 867 with DM. Participants were stratified into tertiles according to SII levels. The primary endpoint was a composite of PCI-related complications, comprising RR and ISR. Associations between SII and outcomes were assessed using multivariable Cox proportional hazards models and restricted cubic spline (RCS) analyses. Over a median follow-up of 60.47 months, 961 patients (46.22%) experienced at least one primary endpoint event. Among patients with DM, those in the highest lnSII tertile (T3) exhibited significantly higher incidences of the composite endpoint and repeat revascularization than those in the T1 and T2 groups (log-rank test, P <0.05). When modeled as a continuous variable, log-transformed SII was independently associated with the composite endpoint (HR: 1.13, 95% CI: 1.01–1.28) and repeat revascularization (HR: 1.16, 95% CI: 1.03–1.31). These associations remained significant in patients with DM (HR: 1.25 [1.06–1.47] and 1.24 [1.03–1.67], respectively), but not in those without DM ( P > 0.05). Moreover, compared with diabetic patients in the lowest SII tertile, those in the highest tertile had markedly increased risks of the composite endpoint and repeat revascularization after multivariable adjustment (HR: 1.37 [1.09–1.72] and 1.43 [1.13–1.81], respectively). RCS analyses demonstrated a significant linear, dose–response relationship between SII and the primary endpoint in patients with DM (P overall = 0.012; P for nonlinearity = 0.092). Among CCS patients undergoing PCI with follow-up coronary imaging, elevated SII was associated with higher long-term risk of post-PCI adverse outcomes, particularly RR, in patients with diabetes. Given the modest effect size and the retrospective imaging-based design, these findings should be interpreted as hypothesis-generating and require external validation.