The relationship of a novel systemic inflammatory response index with metabolic syndrome: a cross-sectional study in Chinese older adults.

The relationship between the systemic inflammatory response index (SIRI) and the risk of metabolic syndrome (MetS) in older adults remains unclear. This study aimed to explore the association between SIRI and MetS in older adults. This cross-sectional study comprised 14,830 older persons who received health examinations in Hangzhou, China from 2023 to 2025. Sociodemographic characteristics and clinical parameters were collected. Multivariate logistic regression was used to analyze the relationship between SIRI and the risk of MetS. A smooth curve analysis was performed to examine the nonlinear association between SIRI and MetS. A piecewise linear regression model was applied to evaluate the threshold effect, and a recursive algorithm was used to identify the inflection point. After adjustment for age, gender, alcohol consumption, hemoglobin (HGB), and red cell distribution width-coefficient of variation (RDW-CV), multivariable logistic regression analysis showed that SIRI was independently and positively associated with MetS in older adults (OR = 1.14, 95% CI: 1.06-1.22, P < 0.001). A threshold effect was identified at a SIRI level of 0.83. Below this threshold, each 1-unit increase in SIRI was associated with a 56% increased risk of MetS (OR = 1.56, 95% CI: 1.28-1.90, P < 0.001), whereas no significant association was observed above the threshold. Significant interactions were found between SIRI and sex as well as between SIRI and HGB level. SIRI was positively associated with MetS risk in women (OR = 1.24, 95% CI: 1.12-1.37, P < 0.001), but the association was not significant in men. Furthermore, the significant positive association between SIRI and MetS was only evident in the low-HGB group (OR = 1.21, 95% CI: 1.09-1.33, P < 0.001). A nonlinear association was found between SIRI and MetS risk in older adults, with an inflection point at 0.83. At low SIRI levels, even a mild elevation markedly increased risk, pointing to a potential sensitive window where immune-inflammatory status amplifies metabolic disturbance-useful for early risk stratification. Above the inflection point, the association plateaued, suggesting other dominant risk factors may take over. Causality cannot be inferred from this cross-sectional design; prospective cohort studies are needed.