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Background Small airways dysfunction (SAD) is an important treatable trait in persistent asthma but remains poorly captured by conventional spirometry. The (FEV 3-FEV1)/FVC and FEV3/FEV6 ratios have been proposed as novel markers of peripheral airflow limitation.Objective We investigated the relationship between (FEV 3-FEV1)/FVC and FEV3/FEV6 with severe exacerbation frequency and symptom control in patients with moderate-to-severe asthma.Methods Clinical, physiological and biomarker data were collected from adults with GINA-defined moderate-to-severe asthma visiting a specialist severe asthma centre in Scotland. Associations between spirometric and oscillometric parameters with clinically relevant outcomes were retrospectively analysed using multivariate models.Results 294 patients were included in the analysis. The (FEV 3-FEV1)/FVC and FEV3/FEV6 ratios were not significantly associated with asthma symptoms or severe exacerbation rate. Conversely, traditional measures such as FEV1/FVC [adjusted odds ratio (95%CI) 2.11 (1.27,3.50) p<0.01], FEF25–75/FVC [2.11 (1.27,3.50) p<0.01], and oscillometric Rrs5–20 [OR 1.90 (1.14,3.16) p<0.05] were significantly associated with ≥2 exacerbations in the previous year (figure 1). High coefficient of determination (r2) values were observed between FEV1/FVC with (a) (FEV3-FEV1)/FVC r2=0.61 and (b) FEV3/FEV6 r2=0.83, suggesting overlapping aspects of lung function being measured. There was a weak correlation between FEV1/FVC and Rrs5–20 values (r2=0.08) indicating that these parameters assess unique aspects of lung function and mechanics.Abstract M19 Figure 1Adjusted odds ratios (95%CI) between spirometry and oscillometry parameters in relation to exacerbation frequencyConclusion In moderate-to-severe asthma, (FEV 3-FEV1)/FVC does not independently predict clinical outcomes and offers no advantage over conventional lung function assessments.