Abstract
While asthma/COPD overlap (ACO), like both asthma and COPD alone, is not a distinct syndrome, nonetheless it does have features that distinguish it from asthma and COPD only. Similar to the latter obstructive pulmonary disorders, it most likely represents a complex spectrum of diseases comprising several different phenotypes and underlying pathophysiologies. It also not yet clear how ACO is best defined, i.e., by clinical features (age, physiology, bronchodilator responsiveness, symptom variability, history of asthma), biomarkers or a combination of these features. The lack of generally agreed-on diagnostic criteria probably accounts for the marked heterogeneity of the results of published surveys of its prevalence. Until a true consensus is achieved regarding the definition of ACO, it will not be possible to determine with confidence not only its prevalence, but also its natural history (outcomes), its underlying biology or its optimal treatment based on findings from randomized controlled clinical trials focused specifically on patients with well-defined ACO.