Systemic immune–inflammation index and in-hospital mortality in critically ill patients with acute asthma exacerbation

The prognostic value of the systemic immune-inflammation index (SII) in critically ill patients with acute asthma exacerbations remains unclear. This study examined the association between SII and in-hospital mortality. We performed a retrospective cohort study using the MIMIC-IV (v3.1) database, including adult critically ill patients with asthma exacerbations. SII was calculated from laboratory values obtained during the peri-intensive care unit admission period. The primary outcome was in-hospital mortality. Associations were assessed using restricted cubic spline and Cox proportional hazards models, adjusted for confounders. A total of 575 patients were included, with an overall in-hospital mortality of 6.6%. Mortality rates differed significantly across SII tertiles and were highest in tertile 3 (11.5%), compared with tertile 2 (3.6%) and tertile 1 (4.7%) (P = .003). In Cox proportional hazards models, compared with tertile 2, patients in tertile 3 had a significantly increased risk of in-hospital mortality (unadjusted hazard ratio [HR] 2.88, 95% confidence interval [CI] 1.21-6.83, P = .016; adjusted HR 2.82, 95% CI 1.12-7.10, P = .027), whereas no significant difference was observed for tertile 1. Restricted cubic spline analysis based on natural logarithm-transformed (ln-SII) demonstrated a significant nonlinear association with in-hospital mortality (P for nonlinearity < .001), with risk increasing markedly at higher ln-SII levels. Elevated SII was associated with increased in-hospital mortality and may serve as a simple prognostic marker in critically ill asthma exacerbations.