Does Eosinophilia Mean Inhaled Corticosteroid Therapy in Obstructive Airway Disease? Revisiting a Treatable Trait across Asthma, Chronic Obstructive Pulmonary Disease, and Bronchiectasis

A BSTRACT Eosinophilic airway inflammation has emerged as an important biomarker in obstructive airway diseases and is increasingly used to guide inhaled corticosteroid (ICS) therapy. In asthma, eosinophilia is a well-established hallmark of type 2 inflammation and predicts favorable responsiveness to corticosteroids. In chronic obstructive pulmonary disease (COPD), blood eosinophil count has become a practical biomarker for identifying patients likely to benefit from ICS-containing regimens, particularly among those with frequent exacerbations. However, the interpretation of eosinophilia in bronchiectasis remains controversial, with limited evidence supporting routine ICS use, except in selected eosinophilic phenotypes or coexisting asthma. This viewpoint examines whether eosinophilia should automatically imply ICS therapy across obstructive airway diseases. We review the biological rationale, current evidence, limitations of eosinophils as biomarkers, disease-specific recommendations, and the balance between therapeutic benefits and adverse effects of ICS. While eosinophilia is a useful treatable trait, indiscriminate extrapolation of asthma paradigms to COPD and bronchiectasis may lead to overtreatment and increased risk of pneumonia, fungal infection, and microbiological colonization. Precision medicine approaches that integrate eosinophil counts with clinical phenotype, exacerbation history, and radiological findings are likely to optimize outcomes.