Association of the Systemic Immune-Inflammation Index with CCTA-Derived SYNTAX Scores in Patients with Obstructive Coronary Artery Disease

Background: Systemic inflammation plays an important role in the pathogenesis and progression of coronary artery disease (CAD). The systemic immune-inflammation index (SII) reflects inflammatory and immune status, whereas the platelet-to-lymphocyte ratio (PLR) represents a simpler hematological inflammatory index. This study aimed to compare the cross-sectional associations of SII and PLR with coronary computed tomography angiography (CCTA)-derived coronary anatomical complexity among selected patients with established obstructive CAD. Methods: In this single-center retrospective cross-sectional study, 197 patients who underwent CCTA were screened. Of these, 104 were excluded, including 68 patients with <50% coronary stenosis, leaving 93 patients with CCTA-defined obstructive CAD (≥50% stenosis) in the final cohort. Patients were categorized as having low, intermediate, or high anatomical complexity according to the CCTA-derived SYNTAX I score. Exploratory ROC analysis assessed discrimination between high (≥33) and lower anatomical complexity within this selected obstructive-CAD cohort. The reduced Firth models underwent internal validation using 1000 bootstrap resamples. Results: Eighteen patients had high anatomical complexity. SII correlated more strongly with the SYNTAX I score than PLR (r = 0.59 vs. 0.34). In adjusted linear models, a doubling of SII was associated with a 6.10-point higher SYNTAX I score (95% CI: 3.52–8.68; p < 0.001), whereas PLR was not significantly associated (p = 0.104). In the Firth models, both SII (per 100-point increase: OR = 1.15, 95% CI: 1.07–1.25) and PLR (per 10-point increase: OR = 1.06, 95% CI: 1.01–1.12) were associated with high complexity. The data-derived SII cut-off of ≥850 yielded 83.3% sensitivity and 76.0% specificity. Optimism-corrected AUCs were 0.82 for age plus SII and 0.70 for age plus PLR. Conclusions: In this selected single-center cohort of patients with CCTA-defined obstructive CAD, SII showed a stronger cross-sectional association with concurrently assessed coronary anatomical complexity than PLR. The exploratory ROC estimates apply only to discrimination between high and lower anatomical complexity within the included cohort and do not establish clinical utility.