Fig. 1

Pooled estimates for the effect of spirometric SAO measured using FEV₃/FVC (a) and FEF_25-75 (b) on respiratory symptoms in the BOLD study. Spirometric SAO: Small airways obstruction. Overall: spirometric SAO defined as FEV₃/FVC or FEF_25-75 less than the lower limit of normal (LLN). Male/Female: spirometric SAO as FEV₃/FVC or FEF_25-75 < LLN stratified by sex. Isolated spirometric SAO: FEV₃/FVC or FEF_25-75 < LLN with FEV₁/FVC ≥ LLN. Dyspnoea measured according to mMRC Dyspnoea scale: 0–1 = minimal/no breathlessness, ≥ 2 = significant breathlessness. Chronic cough: cough on most days for 3 months each year. Chronic Phlegm: Phlegm on most days 3 months each year. Wheeze: Wheezing or whistling in the chest at any time in the last 12 months. OR 95% CI: odds ratio with 95% confidence intervals. I² values of 0%, 25%, 50%, and 75% considered no, low, moderate, and high heterogeneity. Covariates in the adjusted model: sex, education level, body mass index, smoking status, accumulated cigarette pack-years, passive smoking, occupational exposure to dust, use of solid fuels for cooking/heating for > 6 months in a lifetime, reported doctor-diagnosed or history of tuberculosis, spirometric restriction, family history of COPD, and for Dyspnoea addition of CVD. The following sites could not be included in the analysis either due to a low number of participants reporting respiratory symptoms or singularity in the data: Benin (Sémé-Kpodji), Norway (Bergen), Malawi (Blantyre), China (Guangzhou), Germany (Hannover), Cameroon (Limbe), India (Mumbai) (Mysore), Austria (Salzburg), Tunisia (Sousse), Australia (Sydney), Albania (Tirana), Sweden (Uppsala)