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Rationale Eosinophilic exacerbations in asthma and COPD are characterized by small airway dysfunction (SAD) and heterogeneous recovery trajectories. Impulse oscillometry (IOS), a non-effort-dependent technique, enables detailed assessment of airway resistance and reactance, providing insights into lung mechanics during resolution.Methods This analysis compared IOS measurements between early and late recovery visits in 27 patients enrolled in the Acute Exacerbations Treated with Benralizumab (ABRA) trial. Eligible patients were those experiencing acute eosinophilic exacerbations, defined by a blood eosinophil count >300 cells/µL, and were randomized to receive one of three treatment strategies: benralizumab, prednisolone, or a combination of both. IOS parameters, including inspiratory, expiratory, and total resistance (R INSP5, REXP5, RTOT5), and reactance metrics (XINSP5, XEXP5, XTOT5) were collected.Results Patients treated with benralizumab showed significant differences when compared to the prednisolone group in X EXP5 (-1.902 vs -9.669 cmH2O/L/s, p=0.044), and XTOT5 (-1.625 vs -7.367 cmH2O/L/s, p=0.046), suggesting a decline in SAD recovery. In addition, benralizumab and combination therapy exhibited downward trends in RINSP5, REXP5 and RTOT5, indicating improvements in peripheral resistance.Abstract P157 Figure 1Conclusions IOS revealed distinct patterns of airway recovery following eosinophilic exacerbations. Benralizumab alone or in combination was associated with consistent improvements in reactance parameters, reinforcing previously observed findings, 1 while oral prednisolone alone appeared less effective in restoring peripheral airway function. One possible explanation may be that benralizumab improves SAD more than prednisolone due to its faster depletion of airway eosinophils, leading to quicker reduction of inflammation.2 These results highlight the value of IOS as a sensitive method for identifying the resolution of SAD and for tracking the mechanisms underlying responses to steroid and biologic treatments.References Ramakrishnan S, et al. Treating eosinophilic exacerbations of asthma and COPD with benralizumab (ABRA): a double-blind, double-dummy, active placebo-controlled randomised trial. The Lancet Respiratory Medicine 2025;13(1):59–68.Pelaia C, et al. (2018). Benralizumab: from the basic mechanism of action to the potential use in the biological therapy of severe eosinophilic asthma. BioMed Research International 2018(1):4839230.