Epidemiology of asthma-chronic obstructive pulmonary disease overlap (ACO).

The term "asthma-COPD overlap" (ACO) has been applied to the condition in which a person has persistent airflow limitation with clinical features of both asthma and COPD. The certain definition and diagnostic criteria for ACO have not yet been established, and ACO prevalence has varied widely in studies: from 0.9% to 11.1% in the general population, from 11.1% to 61.0% in asthma patients, and from 4.2% to 66.0% in COPD patients. Furthermore, the frequency of exacerbations and prognosis in ACO patients have not been clearly demonstrated. Although ACO consists with several subgroups of patients with distinct clinical and pathophysiological features, it would be important to propose a standardized definition of and/or diagnostic criteria for ACO based on biomarkers and objective measures, even if it is tentative. It may lead cohort studies with large population or clinical trials around the world.