The modified Medical Research Council (mMRC) dyspnoea scale is a measure of breathlessness severity recommended by guidelines and utilised as an inclusion criterion or endpoint for clinical trials. No studies have been conducted to validate the categorical descriptors against the dyspnoea severity grade.

This study utilised cognitive interviews (Think Aloud method) to assess the content validity of the mMRC scale among 16 participants (13 with cardiac/respiratory disease). Participants were recruited to achieve representation across a variety of demographic factors. Interviews were conducted remotely via video conferencing and participants were presented with all 5 mMRC descriptors on screen in random order then asked to rank the statements “in order from the best breathing to the worst breathing”.

Mean age of participants was 57 years (range 22–84 years). Eleven had multimorbidity (≥2 comorbidities) including COPD, asthma, lung cancer, lung infection, interstitial lung disease, heart failure, depression, and anxiety. Length of time with breathlessness ranged between 2 weeks and >25 years. The median rank of the mMRC grade descriptors was concordant for mMRC grades 0, 1 and 4 but not grades 2 and 3. Even so, substantial heterogeneity was found in the distribution of responses for mMRC grade 0.

Our study found substantial heterogeneity in participant grading of the mMRC descriptors, particularly for grades 0, 2 and 3, indicating that mMRC might not be a good discriminator of difference or change in dyspnoea severity. This study demonstrates the importance of content validation even for long-established PROs like mMRC.