A century of increasing lung function and its implications for the diagnosis of lung disease: Results from 243,465 European adults across ten population-based studies

Background: During the last century, socioeconomic and scientific advances changed the health and physique of European populations. Accompanying improvements in lung function, if unrecognised, could lead us to misclassify lung function measurements, and hence diseases such as Chronic Obstructive Pulmonary Disease (COPD). We investigated how population lung function changed with birth year across the last century and how such change may influence lung function interpretation. Methods: We included 243,465 Europeans from ten population-based studies, aged 20-95 years, born between 1884 and 1996. Forced expiratory volume in one second (FEV1) and forced vital capacity (FVC), were measured between 1965 and 2016. We used meta-regression to investigate how FEV1, FVC and the FEV1/FVC ratio changed with birth year. To substantiate our findings, we used linear regression to investigate how the FEV1 and FVC values, predicted by 32 reference equations, published between 1961 and 2015, changed with estimated birth year. Findings: Average European FEV1 and FVC values increased substantially with birth year across the last century. After accounting for height, smoking behaviour, and other co-factors, FEV1 increased by 4.8 mL/birth year (95% confidence interval [CI]:2.6-7.0; P<0.0001) and FVC by 8.8 mL/birth year (95% CI:5.7-12.0; P<0.0001). We found corroboratory birth year-related increases in the FEV1 and FVC values predicted by published reference equations. Whereas FEV1 and FVC increased with advancing birth year, the FEV1/FVC ratio decreased by 0.11 per 100 birth years (95% CI:0.09-0.14; P<0.0001). Interpretation: Average height adjusted European FEV1 and FVC increased with birth year across the last century, causing population values to progressively exceed previously predicted values. Concurrently, the FEV1/FVC ratio decreased. If current diagnostic parameters remain unchanged, the identified shifts in European values will allow the easier fulfilment of diagnostic criteria for COPD, but the systematic underestimation of lung disease severity.