Association between reduced mid-expiratory flow and early airway impairment in smokers with preserved spirometry: a cross-sectional analysis

Aims: To determine whether active smokers with preserved post-bronchodilator spirometry (FEV₁/FVC ≥ lower limit of normal [LLN]) and normal thoracic computed tomography (CT) show lower FEF₂₅₋₇₅ values and a higher prevalence of isolated small airway dysfunction than never-smokers, and to assess PEF-FEV₁ agreement for Global Initiative for Chronic Obstructive Lung Disease (GOLD) severity staging.Methods: We analysed spirometry data from 286 consecutive adults (≥40 years) at a single pulmonary function laboratory. Two chest radiologists independently reviewed all thoracic CT scans (ICC=0.95). Logistic regression included FVC z-score as a prespecified covariate to account for the volume-dependence of FEF₂₅-₇₅; model stability was confirmed by penalised regression and bootstrap validation.Results: Among 244 participants with preserved FEV₁/FVC (136 smokers, 108 never-smokers), FEF₂₅₋₇₅ was lower in smokers (76.2±26.1% vs 93.5±21.9%; p