Association between systemic immune-inflammation index and chronic bronchitis: NHANES 2001–2018

The systemic immune-inflammation index (SII) is a newly identified marker of inflammation., and the relationship between chronic bronchitis (CB) and inflammation is closely associated. However, the influence of SII on CB remains unclear at present.This cross-sectional study was conducted using data from individuals with complete SII and CB records from the 2001-2018 National Health and Nutrition Examination Survey (NHANES). Binary weighted logistic regression was employed to investigate the relationship between SII and CB risk. Additionally, restricted cubic spline regression models and segmented regression models were used to examine nonlinear relationships and threshold effects. Receiver operating characteristic (ROC) curves were adopted to evaluate the predictive value of SII for CB. Stratified analysis was adopted to assess the association between SII and CB in different populations. After adjusting for all covariables, there was a significant positive relevance observed between log-transformed SII (log (SII)) with CB (OR = 1.52, 95% CI: 1.27-1.82, P < 0.001). A nonlinear dose-response relationship with the threshold of 8.14 was observed between log (SII) and CB risk. When log (SII) exceeded 8.14, each unit increase in log (SII) was associated with a 1.31-fold increase in the risk of CB (OR = 1.31, 95% CI: 1.22-1.40, P < 0.001). Furthermore, ROC curves revealed strong predictive capability of SII for CB (AUC = 0.729). Elevated SII levels are associated with an increased prevalence of CB. Furthermore, a non-linear association exists between SII and the increased risk of CB.