The systemic immune-inflammatory index (SII), calculated from platelet, neutrophil, and lymphocyte counts, is a novel marker of inflammation potentially relevant to cardiac disease. While cardiac biomarkers are used clinically, the relationship between SII and these markers in the general population is unclear. We analyzed data from 3206 US adults (aged 20–85 years) in the 1999–2004 National Health and Nutrition Examination Survey (NHANES). Multivariable linear regression models, adjusted for demographics, comorbidities, and lifestyle factors, assessed cross-sectional associations of SII with cardiac biomarkers. Nonlinearity was examined using restricted cubic splines. Subgroup analyses explored effect modification. SII showed a consistent positive association with C-reactive protein (CRP) levels across all models (All P 55 years (β = 0.17; 95% CI 0.014, 0.33), and in those with myocardial infarction (MI) (β = 0.82; 95% CI 0.22, 1.43) or angina pectoris (AP) (β = 0.73; 95% CI 0.19, 1.47). The robust SII-CRP association was confirmed in subgroups. Dose–response relationships were supported by spline analyses. SII is persistently associated with CRP levels, supporting its role as an inflammatory indicator. Significant associations between SII and NT-pro BNP were observed only in specific high-risk subgroups (aged > 55 years, MI or AP patients), but not in the fully adjusted general population. These findings suggest SII may have particular relevance in specific cardiovascular contexts.
Liu et al. (2025) studied this question.