Fixed Ratio Versus Lower Limit of Normal: Health Status and Risk Factors for COPD Overdiagnosis

The threshold of the lower limit of the normal range of lung function has been suggested to be more accurate than the 0.7 fixed ratio (FEV₁/FVC < 0.7) for a diagnosis of COPD. We aimed to explore the health status and risk factors of patients overdiagnosed with COPD when using the lower limit of the normal range as a diagnostic reference.

Subjects with COPD diagnosed by a pulmonologist according to guidelines of the Global Initiative for Chronic Obstructive Lung Disease were recruited from October 2016 to April 2018. Overdiagnosed COPD was defined as FEV₁/FVC that meets the criterion of the 0.7 fixed ratio but not the the lower limit of the normal range criterion. Spirometry and questionnaires were performed by eligible subjects.

Of the 513 subjects included in the final analysis, 20 (3.9%) were overdiagnosed when using the lower limit of the normal range as the diagnostic reference. The subjects who were overdiagnosed were older, weighed more, had better lung function, lower modified Medical British Research Council scores, and higher St. George's Respiratory Questionnaire and 36-item Short Form Survey scores than the subjects who were correctly diagnosed. Older age, heavier weight, exposure to cooking oil fumes, or a new-built or newly renovated home were associated with an increased risk of overdiagnosis of COPD (age adjusted odds ratio (OR) 1.17, 95% CI 1.09-1.26; weight adjusted OR 1.08, 95% CI 1.03-1.13; exposure to cooking oil fumes adjusted OR 3.00, 95% CI, 1.04-8.68; exposure to new-built or newly renovated home adjusted OR 10.88, 95% CI 1.46-80.87.

The subjects with overdiagnosed COPD had a better health status and lung function than the subjects who were correctly diagnosed. Older age, heavier weight, and exposure to cooking oil fumes or a new-built or newly renovated home were factors associated with the overdiagnosis of COPD. These findings may help reduce overdiagnosis of COPD.