Impact of different definitions of airflow limitation on the prevalence of chronic obstructive pulmonary disease in the general population

Background: Recent surveys report a surprisingly high prevalence of chronic obstructive pulmonary disease (COPD) worldwide. However, there is evidence that the application of a fixed ratio of forced expiratory volume in 1 s (FEV(1)) to forced vital capacity (FVC) may result in a relevant misclassification of airflow limitation.

Objectives: The definition of airflow limitation does have a significant impact on its prevalence.

Methods: Individual values of lung volumes were assessed in comparison to a reference population by applying either a fixed ratio below 0.7 or the fifth percentile of FEV(1) in relation to FVC. Based on a large-scale population-based survey, reference equations for lung volumes were derived by quantile regression analysis based on 1,809 subjects aged 25-85 years. Both functional definitions were applied to calculate the prevalence of airflow limitation.

Results: The prevalence of airflow limitation was significantly higher when applying the fifth percentile compared to the fixed ratio of 0.7 for all age groups and both sexes (all 6.7 vs. 1.9%; women 6.7 vs. 1.3%; men 6.6 vs. 2.6%). Almost 26% of the subjects with airflow limitation complained of dyspnea, whereas COPD had not been previously diagnosed by a physician in 87%.

Conclusion: The definition of airflow limitation has a major impact on the observed prevalence rates.