Does spirometry really pick up all the people with COPD and should we keep using LLN given issues around this measurement?

Current diagnostic criteria, which includes spirometric evidence in a person with relevant symptoms and risk factors, appear to be aimed mainly at those individuals with well-established disease (Sakhamuri & Seemungal 2019). Screening for COPD has been largely discouraged as it has not been shown to improve outcomes (Webber et al 2022).

GOLD 2022 guidelines advise that spirometry is required to confirm a diagnosis of COPD in the presence of persistent respiratory symptoms. GOLD guidelines suggest a fixed criterion – the presence of a post-bronchodilator FEV1/FVC <0.70 (<70%) – to confirm the presence of airflow limitation. Other guidelines suggest a post-bronchodilator FEV1/FVC ratio less than the lower limit of normal (LLN; estimated from a representative reference population directly or using a related ‘surrogate’ population) as the fixed cut-off does not reflect the influence of sex (women generally score lower on pulmonary function parameters than men) and age (Grafino et al 2021).

The different criteria to define airway obstruction appear to identify patients with COPD with different characteristics (Grafino et al 2021; Miller et al 2018) and more people appear to meet the fixed cut-off criteria for COPD (Grafino et al 2021; Llordes et al 2020; Meteran et al 2017). The FEV1/FVC varies with age (higher in younger individuals and lower in older individuals). Consequently, a fixed cut-off of 0.7 (70%) is susceptible to over-diagnosis of COPD among the elderly and underdiagnosis in those aged <45 years. For certain ethnic groups, current cut-offs may also result in under-diagnosis of COPD (Aiyer et al 2020; Sood et al 2022).

In order to improve diagnostic accuracy, GOLD recommend repeat spirometry if FEV1/FVC is 0.6–0.8 in a person with relevant symptoms and risk factors (GOLD 2022). In a long-term follow-up of patients with COPD (Llordes et al 2020):

• Those with evidence of obstructive disease using fixed ratio criteria but not obstructive using LLN criteria lost more FEV1/FVC over time than those with evidence of obstructive disease using both criteria.
• Those with evidence of obstructive disease using both criteria experienced more hospitalizations and had higher mortality than those with evidence of obstructive disease using fixed ratio criteria but not obstructive using LLN criteria.

This suggests that LLN criteria are more likely to accurately identify those with clinical COPD.

The LLN criteria are increasingly available in the software of most commercially available spirometers and can be used where they are available at relevant. The European Respiratory Society (ERS) offer a tool to enable calculation of various spirometric values including LLN at http://gli-calculator.ersnet.org/.