Two international respiratory bodies have voiced concern over persistent under-use of spirometry in the diagnosis of COPD, driven largely by different diagnostic criteria.
A joint statement from the Global Initiative for Chronic Obstructive Lung Disease (GOLD) and the Global Lung Function Initiative (GLI) seeks to bring clarity on which threshold should be used to define airflow limitation and demonstrate obstruction in COPD patients, in a bid to end an “unproductive debate” that has lasted more than 30 years.
GOLD recommends use of a fixed ratio – defined as a post-bronchodilator FEV1/FVC < 0.7 – to define COPD-related airflow obstruction and confirm diagnosis in people whose medical history suggests COPD.
By contrast, GLI recommends a lower limit of normal (LLN) value for the FEV1/FVC ratio to define airflow obstruction in a wider patient population. The LLN is based on the lower boundary of measurements from a healthy reference population.
“Neither threshold is intrinsically right or wrong,” the authors wrote in the statement, published in European Respiratory Journal [link here]. “Each threshold for defining airflow obstruction evolved to solve different problems, and discordance between the criteria reflects fundamentally different goals.”
“The threshold proposed by GOLD was intended to be a pragmatic approach for confirmation of the diagnosis of COPD in everyday practice. The threshold proposed by GLI intends to reflect the variability in lung function measures observed between individuals in populations,” they said.
Crucially, GOLD and GLI agree that the LLN can be used to identify airflow obstruction more broadly in populations, but that “in the appropriate clinical context (e.g. a symptomatic patient with a medical and exposure history suggestive of COPD), recommend that the fixed ratio is used to identify COPD-related airflow obstruction and confirm a diagnosis of COPD”.
The groups note that the fixed ratio is simpler but doesn’t account for potential age-related lung function changes, and may identify fewer younger and more older individuals with airflow obstruction.
The LLN considers age-related changes in lung function, which reflect the loss of tissue elasticity. Yet, it depends on the characteristics of the reference group and often assumes a “healthy” base population.
Moreover, by considering age-related decline as physiologically normal, the use of the LLN can diagnose more younger and fewer older individuals with airflow obstruction. If LLN is used in the absence of references to symptoms or exposure to COPD risk factors, it will “falsely label some healthy younger individuals as having COPD,” the authors noted.
However, leading UK respiratory scientist and clinician Professor David Halpin, a co-author of the statement, said the debate may be overstated. “There is uncertainty in the measurement and normal values that means the argument about fixed ratio or LLN is largely irrelevant,” he told the limbic.
“Discrepant values will almost always fall within the margin of error.”
GLI and GOLD agree that differences in their recommendations on thresholds of ‘abnormality’ are outweighed by shared concerns on the under-use of spirometry. In practice, only a small proportion of people are classified differently by the two approaches, they said.
Importantly, the groups also agree that while FEV1/FVC measured by spirometry is the most appropriate biomarker of obstruction, spirometry alone is not sufficient to make a diagnosis of COPD. As such, neither GOLD nor GLI recommend the diagnosis or labelling of patients with disease in the absence of a clinical history.
Both the fixed ratio and LLN can be useful for confirming disease in people who have respiratory history and a significant history of exposure. However, studies have shown that using the fixed ratio can identify early disease in people at risk of COPD before the LLN is able to detect overt obstruction, the authors noted.
“The [joint article] clarifies the use of the fixed ratio (0.7) to diagnose airflow obstruction in COPD rather than the LLN – which has been a source of great debate and confusion,” said Professor Halpin, of the University of Exeter .
The statement contained a number of important messages, but “the main aim is to get more spirometry done!”, he said.