Clinical Implications of a 1.0% Blood Eosinophil Threshold in Acute Exacerbations of COPD: a retrospective observational study

Abstract Background Eosinophils play an increasingly recognized role in chronic obstructive pulmonary disease (COPD). However, the optimal threshold for blood eosinophil (EOS) counts remains controversial. This study aimed to identify a clinically meaningful EOS threshold in acute exacerbations of COPD (AECOPD) and characterize patients with elevated EOS. Methods In this retrospective observational study, 1,406 AECOPD patients hospitalized were enrolled. Clinical data were analyzed, and receiver operating characteristic (ROC) curves were used to evaluate the optimal EOS threshold based on hospitalization costs. Patients were stratified by EOS levels (1.0%, 1.25%, 1.50%, 1.75%, 2.0%), and clinical characteristics were compared. Results Among enrolled patients, 83.3% were male, averaging 66.16 years, with 71.5% having a smoking history, predominantly GOLD stage II. Median hospital stay was 8 days, costing 9412.58 RMB. ROC analysis determined that a 1.0% EOS cutoff (AUC = 0.652, 95% CI: 0.626–0.677; Youden index = 0.2581, Z = 9.73, P < 0.001) is the optimal threshold. Patients with EOS ≥ 1.0% were younger, incurred lower costs, and had shorter stays. They showed reduced sputum neutrophils and lymphocytes, elevated eosinophils, higher albumin, lower urea nitrogen and HbA1c, and less diabetes. Improved FEV 1 % and PEF, fewer ICU admissions, and reduced antibiotic and corticosteroid use were also observed. Conclusion This study identified that a cutoff value of 1.0% for peripheral blood EOS percentage more effectively reflects the clinical characteristics of patients with AECOPD associated with elevated peripheral blood EOS levels.