Association of Systemic Immune-Inflammation Index With Stroke and Mortality Rates: Evidence From the NHANES Database.

Objective: This study aimed to examine the association of the systemic immune-inflammation index (SII) with stroke and mortality rates using data from the National Health and Nutrition Examination Survey (NHANES).

Methods: A cross-sectional study was conducted using the aggregated data from 5 cycles (2009 to 2018) of NHANES. SII was the independent variable, and stroke was the dependent variable. Weighted logistic regression models were employed to analyze their relationship. The nonlinear association between SII and stroke was examined using the restricted cubic spline (RCS) method in subgroups stratified by smoking status, hypertension, and dietary inflammatory index. Weighted Kaplan-Meier curves and Cox regression analysis were used to investigate the association of SII with all-cause mortality and cardiovascular disease (CVD) mortality.

Results: A total of 22,107 samples were included in this study. Weighted logistic regression analysis showed a significant correlation between SII and stroke (OR: 1.53, 95% CI: 1.22-1.92, P<0.001). The stratified analysis revealed that interactions of smoking status and hypertension with SII, respectively, had significant impacts on stroke risk. A remarkable positive link between SII and stroke risk (OR>1, P<0.05) was observed in the crude model (unadjusted for confounding factors), model I (adjusted for demographic characteristics), and model II (adjusted for all confounding factors). RCS analysis displayed a remarkable nonlinear positive correlation between SII and stroke risk only in the "now smoking" population (P-nonlinear<0.05) after adjusting for all confounding factors. In the overall sample population, Kaplan-Meier curves indicated that individuals in the highest quartile of SII had the highest risk of all-cause mortality and CVD mortality (log-rank test P<0.05). Samples with proinflammatory dietary habits had considerably higher risks of all-cause mortality and CVD mortality compared with those with anti-inflammatory dietary habits (log-rank test P<0.05). Multivariable-adjusted Cox regression models showed significantly increased all-cause mortality and CVD mortality rates in the highest quartile of SII compared with the lowest quartile.

Conclusions: SII levels were considerably positively linked to stroke risk, particularly in the "now smoking" population. Moreover, elevated SII levels increased the risk of all-cause mortality and CVD mortality in the overall population. On the basis of these findings, we recommend incorporating smoking cessation measures into stroke risk reduction strategies.