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.