Prevalence of small airways obstruction and its risk factors in the multinational Burden of Obstructive Lung Disease (BOLD) study

Background: Small Airways Obstruction (SAO) is a common feature of obstructive lung diseases. There is limited research on SAO, its global prevalence and risk factors. Methods: Using data from 41 sites in the cross-sectional Burden of Obstructive Lung Disease study (N=26,448), we defined SAO as either: 1) mean forced expiratory flow rate between 25% and 75% of the forced vital capacity (FEF25-75) less than lower limit of normal (LLN), or 2) forced expiratory volume in three seconds to forced vital capacity ratio (FEV3/FVC) less than the LLN. We estimated the prevalence of pre- and post-bronchodilator SAO for each site. To identify risk factors for SAO, we performed multivariable regression analyses within each site, and pooled estimates using random effects meta-analysis. Findings: Prevalence of pre-bronchodilator SAO ranged from 5% (34/624) in Tartu (Estonia) to 34% (189/555) in Mysore (India) for FEF25-75, while for FEV3/FVC it ranged from 5% (31/667) in Riyadh (Saudi Arabia) to 31% (287/981) in Salzburg (Austria). Prevalence of post-bronchodilator SAO was universally lower. Risk factors associated with FEV3/FVC included increasing age, low body mass index, active and passive smoking, low level of education, working in a dusty job for more than 10 years, and previous tuberculosis. Results were similar for FEF25-75, except for increasing age, which was associated with reduced odds of SAO. Interpretation: Despite the wide geographical variation, SAO is common and more prevalent than chronic airflow obstruction worldwide. SAO shows the same risk factors as chronic airflow obstruction. However, further research is required to investigate whether it also associates with respiratory symptoms and lung function decline. Funding: National Heart and Lung Institute; Wellcome Trust (085790/Z/08/Z).