Vital capacity and COPD: the Swedish CArdioPulmonary bioImage Study (SCAPIS)

BACKGROUND: Spirometric diagnosis of chronic obstructive pulmonary disease (COPD) is based on the ratio of forced expiratory volume in 1 second (FEV(1))/vital capacity (VC), either as a fixed value <0.7 or below the lower limit of normal (LLN). Forced vital capacity (FVC) is a proxy for VC. The first aim was to compare the use of FVC and VC, assessed as the highest value of FVC or slow vital capacity (SVC), when assessing the FEV(1)/VC ratio in a general population setting. The second aim was to evaluate the characteristics of subjects with COPD who obtained a higher SVC than FVC. METHODS: Subjects (n=1,050) aged 50–64 years were investigated with FEV(1), FVC, and SVC after bronchodilation. Global Initiative for Chronic Obstructive Lung Disease (GOLD) COPD(FVC) was defined as FEV(1)/FVC <0.7, GOLDCOPD(VC) as FEV(1)/VC <0.7 using the maximum value of FVC or SVC, LLNCOPD(FVC) as FEV(1)/FVC below the LLN, and LLNCOPD(VC) as FEV(1)/VC below the LLN using the maximum value of FVC or SVC. RESULTS: Prevalence of GOLDCOPD(FVC) was 10.0% (95% confidence interval [CI] 8.2–12.0) and the prevalence of LLNCOPD(FVC) was 9.5% (95% CI 7.8–11.4). When estimates were based on VC, the prevalence became higher; 16.4% (95% CI 14.3–18.9) and 15.6% (95% CI 13.5–17.9) for GOLDCOPD(VC) and LLNCOPD(VC), respectively. The group of additional subjects classified as having COPD based on VC, had lower FEV(1), more wheeze and higher residual volume compared to subjects without any COPD. CONCLUSION: The prevalence of COPD was significantly higher when the ratio FEV(1)/VC was calculated using the highest value of SVC or FVC compared with using FVC only. Subjects classified as having COPD when using the VC concept were more obstructive and with indications of air trapping. Hence, the use of only FVC when assessing airflow limitation may result in a considerable under diagnosis of subjects with mild COPD.