Background Chronic airflow obstruction is a key characteristic of COPD. We aimed to investigate whether isolated small airways obstruction can predict chronic airflow obstruction later in life.

Methods We used longitudinal data from the multinational Burden of Obstructive Lung Disease study. We recruited a representative sample of adults, aged 40 years or older, and performed spirometry at two time points. We defined isolated small airways obstruction as a pre-bronchodilator mean forced expiratory flow rate between 25% and 75% of the forced vital capacity (FEF_25?75) less than the lower limit of normal (<LLN) in presence of a normal forced expiratory volume in 1 s to forced vital capacity ratio (FEV₁/FVC). We also used the forced expiratory volume in 3 s to forced vital capacity ratio (FEV₃/FVC) to define small airways obstruction. We defined chronic airflow obstruction as a post-bronchodilator FEV₁/FVC <LLN. We used mixed effects logistic regression to model the association between baseline isolated small airways obstruction and chronic airflow obstruction at follow-up. We evaluated predictive performance using the area under the curve (AUC) and Brier scores.

Results Data on 3,856 participants were analysed (median follow-up 8.4 years). Chronic airflow obstruction was more likely to develop in participants with isolated small airways obstruction at baseline (FEF_25-75 <LLN: OR=5.71, 95%CI 2.53-12.88; FEV₃/FVC <LLN: OR=2.69, 95%CI 1.87-3.89). FEV₃/FVC predicted chronic airflow obstruction better than FEF_25-75 (AUC: 0.76 vs 0.74; Brier score: 0.08 vs 0.10).

Conclusions Spirometric measurements of small airways function can be used as early markers of future obstructive lung disease.