An inverted U-shaped curve relationship between systemic immune-inflammation index and stroke in older adults: A population-based study

BACKGROUND: The present research sought to examine potential links connecting the systemic immune-inflammation index (SII), an emerging biomarker of inflammatory status, with stroke among aging individuals. MATERIALS AND METHODS: Researchers performed a cross-sectional analysis utilizing datasets obtained from the National Health and Nutrition Examination Survey (NHANES) spanning the years 2017 to 2020. Three distinct analytical approaches-multivariable regression, smoothing curve fitting, and threshold effect analyses-were systematically applied to investigate potential correlations between SII and stroke. RESULTS: When SII was analyzed as a categorical variable, the minimally adjusted model showed a reduction in stroke occurrence in the middle SII group compared to the lowest group according to logistic regression analysis (OR = 0.515, 95 % CI: 0.275-0.967, p = 0.039). Smoothed curve fitting and threshold effects analysis were performed after adjusting for age, race, education, heart disease, hypertension, hyper lipidaemia, diabetes and smoking status. The results showed that when SII was less than 0.894 (10⁶cells/µl), the risk of stroke increased by 181.3 % for each unit increase in SII (OR = 2.813, 95 % CI: 1.515-5.224, p = 0.001). Meanwhile, when SII was > 0.894 (10⁶ cells/μl), the risk of stroke was reduced by 65.1 % for every 1 unit increase in SII (OR = 0.349, 95 % CI: 0.135-0.902, p = 0.030). CONCLUSION: An inverted U-shaped association exists between SII and stroke in older adults. Although this study is cross-sectional and cannot establish a definitive causal relationship between SII and stroke, it provides important evidence for future research to better identify high-risk populations for stroke.