Small airway dysfunction as a functional barrier to spirometry-defined clinical remission in severe asthma treated with biologic therapies.

Background and objective: Small airway dysfunction (SAD) is a frequent yet underrecognized feature of severe asthma that may influence treatment outcomes and remission. This study evaluated the prevalence of SAD using impulse oscillometry (IOS) and spirometry, and its impact on clinical remission on biologic therapy. Methods: Severe asthma patients were assessed for SAD by IOS, spirometry, and combined modalities. Baseline characteristics, biomarkers, and clinical outcomes were compared between patients with and without SAD. Changes after 1 year of biologic therapy were analyzed, and associations between SAD and clinical remission criteria (CR) were evaluated. Results: SAD was most frequently detected by IOS (66.9%), followed by spirometry (54.9%). Patients with SAD were significantly older and had higher rates of allergic rhinitis, diabetes, and hypertension (all P < 0.05). Baseline pulmonary function showed greater airflow limitation and peripheral airway resistance in the SAD group (P < 0.001). Biologic therapy significantly improved asthma control, reduced exacerbations, and enhanced FEV Conclusion: SAD is common in severe asthma and is associated with older age, metabolic and allergic comorbidities, and worse baseline lung function. While biologic therapy yields substantial clinical and functional improvement, the persistence of SAD predicts a lower likelihood of CR and distinct biomarker dynamics, underscoring the importance of assessing small airway function in personalized asthma management.