Chronic obstructive pulmonary disease (COPD) is one of the leading causes of disability and death in both the developed and developing worlds,1^–3 and it is largely underdiagnosed.4

While it is generally accepted that the ratio between forced expiratory volume in 1 s (FEV₁) and forced vital capacity (FVC) or slow vital capacity (SVC) is the most important measure that characterises airflow obstruction,5 6 there is still no consensus on what the best definition of COPD should be.

In an attempt to simplify the diagnosis of COPD, improve the detection of the disease in primary care and standardise methods to measure the prevalence of COPD in different countries, the Global Initiative for Chronic Obstructive Lung Disease (GOLD) has defined COPD as a post-bronchodilator FEV₁/FVC ratio <0.70.5

As the FEV₁/FVC ratio decreases with age, this fixed ratio has been criticised as it may overdiagnose the disease in elderly subjects7 and underdiagnose it in young adults.8 For this reason, the ATS/ERS guidelines on lung function testing9 propose using the lower limit of normality (LLN) instead of a fixed cut-off to define COPD. LLN is the lower fifth percentile of the frequency distribution of the FEV₁/FVC ratio in a healthy population of a given sex and age. The guidelines also point out that the SVC could be more accurate than FVC to diagnose airflow obstruction.

A hot debate between the supporters of the two different points of view is currently ongoing, as can …