Abstract
BACKGROUND: The diagnosis of chronic obstructive pulmonary disease is based on the presence of persistent respiratory symptoms and chronic airflow limitation (CAL). CAL is based on the ratio of forced expiratory volume in 1 s to forced vital capacity (FEV(1):FVC) after bronchodilation, and FEV(1):FVC less than the fifth percentile is often used as a cut-off for CAL. The aim was to investigate if increasing percentiles of FEV(1):FVC were associated with any respiratory symptom (cough with phlegm, dyspnoea or wheezing) in a general population sample of never-smokers and ever-smokers. METHODS: In a cross-sectional study comprising 15 128 adults (50-64 years), 7120 never-smokers and 8008 ever-smokers completed a respiratory questionnaire and performed FEV(1) and FVC after bronchodilation. We calculated their z-scores for FEV(1):FVC and defined the fifth percentile using the Global Lung Function Initiative (GLI) reference value, GLI(5) and increasing percentiles up to GLI(25). We analysed the associations between different strata of percentiles and prevalence of any respiratory symptom using multivariable logistic regression for estimation of OR. RESULTS: Among all subjects, regardless of smoking habits, the odds of any respiratory symptom were elevated up to the GLI(15-20) strata. Among never-smokers, the odds of any respiratory symptom were elevated at GLI(<5) (OR 3.57, 95% CI 2.43 to 5.23) and at GLI(5-10) (OR 2.57, 95% CI 1.69 to 3.91), but not at higher percentiles. Among ever-smokers, the odds of any respiratory symptom were elevated from GLI(<5) (OR 4.64, 95% CI 3.79 to 5.68) up to GLI(≥25) (OR 1.33, 95% CI 1.00 to 1.75). CONCLUSIONS: The association between percentages of FEV(1):FVC and respiratory symptoms differed depending on smoking history. Our results support a higher percentile cut-off for FEV(1):FVC for never-smokers and, in particular, for ever-smokers.