High systemic immune inflammation index values are associated with prolonged length of hospital stay in patients with acute exacerbation of chronic obstructive pulmonary disease: a retrospective cohort study.

Objectives: This study aimed to investigate the association between the Systemic Immune Inflammation Index (SII) and the prolonged length of hospital stay (PLOS) in patients with acute exacerbation of chronic obstructive pulmonary disease (AECOPD). Methods: A retrospective analysis was conducted involving 986 patients aged ≥ 40 years with AECOPD admitted to Kailuan General Hospital between January 2018 and December 2024. PLOS was defined as a stay exceeding 7 days. Complete blood counts were collected within 24 h of admission to calculate the SII, which was the log-transformed and denoted as In-SII. Logistic regression analysis was employed to compare the predictive value of In-SII and In-NLR (neutrophil-to-lymphocyte ratio) for PLOS in patients with AECOPD. Additionally, restricted cubic splines (RCS) and decision curves analysis (DCA) were utilized to explore the nonlinear relationship and clinical net benefit between In-SII and PLOS in patients with AECOPD. Results: In-SII was an independent risk factor for PLOS in patients with AECOPD (odds ratios for Model 1, Model 2, and Model 3 were 1.527, 1.294, and 1.496, respectively; p < 0.05). Its predictive performance is superior to In-NLR. According to RCS curves, there was a linear association between In-SII and PLOS in patients with AECOPD (Model 1: p for nonlinear = 0.664; Model 2: p for nonlinear = 0.663; Model 3: p for nonlinear = 0.571). Additionally, DCA indicated a significant net clinical benefit when the In-SII threshold ranged from 0.41 to 0.80. Conclusion: High SII serves as an independent risk factor for PLOS in patients with AECOPD. This indicated that patients with AECOPD exhibiting high SII levels have poorer outcomes, necessitating earlier implementation of more robust intervention measures and close monitoring of disease progression.