Concordance between FVC and FEV₆ for identifying chronic airflow obstruction and spirometric restriction in the Burden of Obstructive Lung Disease (BOLD) study

Introduction We investigated whether the forced expiratory volume in 6 s (FEV₆) can be used as a surrogate for the forced vital capacity (FVC). Methods The Burden of Obstructive Lung Disease is a multinational cohort study. At baseline, data were collected from adults, aged 40 years or older, from 41 sites across 34 countries. Participants from 18 sites were followed-up after a median of 8.3 years. Participants who completed the study core questionnaire and had acceptable post-bronchodilator spirometry were included. We performed receiver operating characteristic analyses to measure the ability of FEV₁/FEV₆ less than the lower limit of normal (LLN) to correctly classify FEV₁/FVC less than the LLN, and FEV₆ less than the LLN to correctly classify FVC less than the LLN. We used multilevel regression analyses to assess the association of discordant measurements with respiratory symptoms, quality of life and lung function decline. Results At baseline, 28 604 participants were included. 53% were female (15 060). 10% (2876) had chronic airflow obstruction for FEV₁/FVC, compared with 9% (2704) for FEV₁/FEV₆. 37% (10 637) had spirometric restriction for FVC, compared with 35% (9978) for FEV₆. The FEV₁/FEV₆ had excellent accuracy in identifying FEV₁/FVC less than the LLN (area under the curve (AUC): 0.90, 95% CI, 0.89 to 0.91, κ coefficient 0.82). The FEV₆ also had excellent agreement in identifying FVC less than the LLN (AUC: 0.95, 95% CI, 0.94 to 0.95, κ coefficient 0.90). Discordant reductions in FEV₁/FEV₆ (1%, 345) and FEV₆ (1%, 309) were associated with greater odds of having respiratory symptoms and a lower physical quality of life. 3870 participants were followed up. Those with discordant reductions in FEV₁/FEV₆ and FEV₆ were more likely to have chronic airflow obstruction and spirometric restriction at follow-up. Conclusions There is strong agreement between the FVC and FEV₆ in the identification of chronic airflow obstruction and spirometric restriction.