COPD is a major burden globally. According to the Global Burden of Disease study, COPD caused 3.2 million deaths in 2015, accounting for 5% of all deaths worldwide, making it the third leading cause of death in the world. The Global Initiative for Chronic Obstructive Lung Disease (GOLD) defines spirometrically confirmed COPD based on a forced expiratory volume during the first second (FEV1) to a forced vital capacity (FVC) ratio smaller than 0.7. The severity of airflow obstruction is further defined through GOLD severity grades based on the ratio of FEV1 to its predicted value, with GOLD 1, 2, 3 and 4 defined around cut-off points of 80%, 50%, and 30%.

While diagnostic and disease management decisions (e.g. therapeutic choices) demand definitions that create distinct categories, the physiological processes underlying COPD act on a continuous scale . For example, it is recognised that patients fall on a continuous spectrum on the three major aspects of COPD: rate of lung function decline, frequency of acute COPD exacerbations and symptom burden , with little correlation between the three. Categorising such a continuous process inevitably results in COPD phenotypes that are numerous, loosely defined, and not always mutually exclusive .