Nonlinear association between the systemic immune-inflammation index and hypertension in overweight and obese Chinese adults: a cross-sectional study of 40,633 participants

Background The systemic immune-inflammation index (SII) reflects systemic inflammatory and immune status, but its association with hypertension among overweight and obese Chinese adults remains unclear. Methods This cross-sectional study included 40,633 adults with a body mass index (BMI) ≥ 24 kg/m 2 who underwent health screening at Henan Provincial People's Hospital from 2018 to 2023. SII was calculated from neutrophil, platelet, and lymphocyte counts and natural-log transformed; the resulting variable was denoted as logSII. Multivariable logistic regression, generalized additive models, two-piecewise regression, bootstrap validation, subgroup analysis, receiver operating characteristic analysis, and decision curve analysis were performed. Results After adjustment for the prespecified measured covariates, each one-unit increase in logSII was associated with higher hypertension odds (odds ratio [OR] = 1.22, 95% confidence interval [CI]: 1.16–1.28), and participants in the highest quartile had greater odds than those in the lowest quartile (OR=1.26, 95% CI: 1.18–1.35). A nonlinear association was observed, with hypertension odds increasing below an estimated breakpoint and subsequently reaching a plateau; an exploratory, data-driven breakpoint was estimated at logSII = 6.37, which represents a statistical feature of the observed association rather than a validated biological or clinical threshold. Below this threshold, higher logSII was associated with increased hypertension odds (OR=1.32, 95% CI: 1.23–1.41), whereas no significant association was observed above it (OR=1.01, 95% CI: 0.85–1.19). Bootstrap analysis yielded a median threshold of 6.36 (interquartile range, 6.27–6.48). No clinically meaningful improvement in discrimination was observed after adding SII to the clinical model, indicating limited incremental value beyond established clinical variables, and no clear superiority over the neutrophil-to-lymphocyte ratio (NLR) or systemic inflammation response index (SIRI). Conclusions Among overweight and obese Chinese adults, logSII was associated with prevalent hypertension. Because SII and hypertension were assessed concurrently, temporality cannot be determined, reverse causation remains possible, and causal or predictive inference is not supported. The estimated breakpoint is an exploratory statistical finding rather than a clinically applicable cutoff and requires prospective multicenter external validation.