Overlapping mechanisms and novel biomarkers define asthma and chronic obstructive pulmonary disease (COPD) as a spectrum of airway diseases.

Chronic obstructive pulmonary disease (COPD) and asthma are prevalent obstructive airway diseases with shared symptoms (cough, wheezing, and dyspnea) and treatments (inhaled corticosteroids and bronchodilators). The prototypical clinical phenotypes are well-recognized, with asthma as a childhood-onset, episodic, bronchodilator-responsive condition related to atopy and responsive to treatment with inhaled steroids, and COPD as an adult-onset, progressive disorder that develops after years of exposure to cigarette smoke or inhaled biomass smoke, leading to fixed airway obstruction. However, there is well-recognized heterogeneity in the molecular pathophysiology within both diseases, and substantial overlap in proposed pathophysiological mechanisms. Furthermore, overlapping inflammatory responses occur in endotypes of both asthma and COPD. This overlap includes type 2-high asthma and eosinophilic COPD, and a type 3 (TH17) inflammation-driven type 2-low asthma and neutrophilic inflammation in COPD. Recent advances in cross-sectional imaging that permit quantitative evaluation of mucus plugging have demonstrated a high burden of mucus obstruction in both conditions. A portion of individuals who have features of asthma and COPD have been categorized as having asthma-COPD overlap. However, a universally accepted definition of asthma-COPD overlap has yet to be established, and COPD and asthma are widely considered distinct disease entities. Because specific endotypes often dictate the choice and likelihood of response to targeted therapeutics, identifying biomarkers that cross the spectrum of asthma and COPD will be important for successful precision medicine. In this review, we discuss shared features between asthma and COPD and highlight current and emerging molecular and imaging biomarkers that may guide personalized treatment for obstructive airway disease.