Identifying abnormal exertional breathlessness in COPD: comparing mMRC and CAT with CPET

Background: Chronic obstructive pulmonary disease (COPD) management is guided by the respiratory symptom burden, assessed using the modified Medical Research Council (mMRC) scale and/or COPD Assessment Test (CAT).

Research question: What is the ability of mMRC and CAT to detect abnormally high exertional breathlessness on incremental cardiopulmonary cycle exercise testing (CPET) in people with COPD?

Study design and methods: Analysis of people aged ≥40 years with post-bronchodilator FEV₁/FVC<0.70 and ≥10 smoking pack-years from the Canadian Cohort Obstructive Lung Disease study. Abnormal exertional breathlessness was defined as a breathlessness (Borg 0-10) intensity rating > upper limit of normal (ULN) at the symptom-limited peak of CPET using normative reference equations.

Results: We included 318 people with COPD (40% women), age 66.5±9.3 years (mean±SD), FEV₁ 79.5±19.0%predicted; 26% had abnormally low exercise capacity (V'O_2peak <lower limit of normal). Abnormally high exertional breathlessness was present in 24%, including 9% and 11% of people with mMRC=0 and CAT<10, respectively. A mMRC≥2 and CAT≥10 was most specific (95%) to detect abnormal exertional breathlessness, but had low sensitivity of only 12%. Accuracy for all scale cut-offs or combinations was <65%. Compared with 'true negatives', people with abnormal exertional breathlessness but low mMRC and/or CAT scores ('false negatives') had worse self-reported and physiological outcomes during CPET, were more likely to have physician-diagnosed COPD, but were not more likely to have any respiratory medication (37% versus 30%; mean difference 6.1%; 95% confidence interval -7.2 to 19.4; p=0.36).

Interpretation: In COPD, mMRC and CAT have low concordance with CPET and fail to identify many people with abnormally high exertional breathlessness.