Linear association between the systemic immune-inflammation index and all-cause mortality in patients with interstitial lung disease: a retrospective cohort study

Objective: To investigate the association between the systemic immune-inflammation index (SII) and all-cause mortality in patients with interstitial lung disease (ILD). Methods: This retrospective cohort study included 366 patients with ILD. SII was calculated using peripheral blood counts and analyzed as both a continuous and categorical variable based on the optimal cutoff value determined by receiver operating characteristic (ROC) analysis. Kaplan-Meier survival curves were used to compare survival between SII groups. Univariable and multivariable Cox proportional hazards models were applied to evaluate the association between SII and all-cause mortality. Restricted cubic spline (RCS) analysis was performed to assess the dose-response relationship. Subgroup analyses were conducted to examine the robustness of the association. Results: for nonlinearity = 0.305). Subgroup analyses indicated significant associations between higher SII and all-cause mortality among patients aged ≥60 years, females, and those without anti-synthetase syndrome. Conclusion: Elevated SII is independently and linearly associated with increased all-cause mortality in patients with ILD. These findings suggest that SII may have potential clinical value in the assessment and management of patients with ILD.