Beyond Eosinophilia: A Dual Eosinophil Phenotype Model in Bronchiectasis

Background and Objectives: Bronchiectasis exhibits heterogeneous inflammatory features. Although peripheral blood eosinophil counts guide therapy in other airway diseases, their role in phenotyping bronchiectasis remains under-defined. We investigated the clinical, microbiological, and inflammatory profiles of eosinopenic and eosinophilic phenotypes based on stable-state blood eosinophil counts. Materials and Methods: This retrospective cohort study evaluated 163 patients diagnosed with bronchiectasis between December 2020 and October 2024. Patients were stratified into three groups based on stable-state peripheral blood eosinophil counts: eosinopenia (≤100 cells/µL), intermediate eosinophil levels (101-299 cells/µL), and eosinophilia (≥300 cells/µL). Baseline demographics, comorbidities, laboratory data, sputum culture results, radiological scores, and clinical outcomes were compared. Factors associated with exacerbations were evaluated using multivariable logistic regression. Results: Eosinopenia was present in 30.1% of patients, intermediate eosinophil levels in 46.0%, and eosinophilia in 23.9%. Compared with patients in the intermediate group, those with eosinopenia had significantly higher sputum culture positivity (53.1% vs. 21.3%; p < 0.001), more frequent Pseudomonas aeruginosa isolation in stable-state sputum cultures (26.5% vs. 9.3%; p = 0.011), and a higher neutrophil-to-lymphocyte ratio. Hospitalization rates were numerically higher in the eosinophilia group than in the intermediate group (38.5% vs. 18.7%), although the overall between-group difference did not reach statistical significance (p = 0.069). In multivariable analysis, neither eosinopenia nor eosinophilia was significantly associated with exacerbation (adjusted OR 0.88, 95% CI 0.39-2.00; p = 0.760 and adjusted OR 1.51, 95% CI 0.56-4.07; p = 0.418, respectively). The modified Reiff score showed only a borderline association after adjustment (adjusted OR 1.16, 95% CI 0.99-1.35; p = 0.060). Conclusions: Blood eosinophil levels were associated with distinct clinical phenotypes in bronchiectasis. Eosinopenia was characterized by higher NLR and more frequent P. aeruginosa isolation in stable-state sputum cultures, whereas eosinophilia was associated with a distinct clinical profile. Blood eosinophil counts may serve as a practical phenotyping tool rather than a linear prognostic biomarker.