Interpreting blood eosinophil counts in health and obstructive lung disease

Asthma and COPD are prevalent chronic diseases characterised by presence of respiratory symptoms and airflow limitation caused by ongoing inflammatory process in the airways [1, 2]. While the clinical features are reversible or variable over time in asthma [1], they are more persistent and usually progressive over time in COPD [2]. The connection between these two common disorders has been debated thoroughly for many decades with proponents for both "splitting" or "lumping" them into either two distinct or one common entity [3–6]. The clinical heterogeneity of both conditions has also been recognised for a long time, and the fact that some patients show characteristics of both diseases simultaneously resulted in the official introduction of the asthma–COPD overlap label by the Global Initiative for Chronic Obstructive Lung Disease (GOLD) and Global Initiative for Asthma committees in 2014 [7].