Background: The 6-minute walk test (6MWT) is widely used to assess exercise capacity but is currently not useful to assess breathlessness or leg fatigue as the scores do not account for the distance walked (6MWD). To enable symptom evaluation, we aimed to develop normative reference equations for breathlessness and leg fatigue intensity expressed as modified Borg (mBorg)/6MWD ratios, and to validate them in people with chronic obstructive pulmonary disease (COPD).

Methods: Analysis of people aged ? 40 years who performed two 6MWTs in the Canadian Cohort Obstructive Lung Disease (CanCOLD) study; a healthy cohort (n=361; mean age 67.5±9.3 years; 52.9% male) with normal 6MWD and lung function; and a COPD cohort (n=156; mean age 66.2± 9.0 years; 56.4% male; 39.1% GOLD1, 49.4% GOLD2, 9.6% GOLD3, and 1.9% GOLD4). The mBorg score was calculated as the modified Borg 0-10 category ratio breathlessness or leg fatigue score +1 (range 1?11), to avoid zeros and yield ratios proportional to the symptom score and 6MWD^-1.

Results: Normative reference equations for breathlessness and leg fatigue mBorg/6MWD ratios were developed using multivariable linear regression, accounting for age, body mass or body mass index. In the COPD cohort, abnormal breathlessness and leg fatigue (mBorg/6MWD>upper limit of normal [ULN]) showed strong concurrent validity with higher GOLD stage, Medical Research Council (MRC) breathlessness, and COPD Assessment Test scores.

Conclusion: The mBorg/6MWD ratio is presented and validated as the first method to assess exertional breathlessness and leg fatigue response to the 6MWT in people with COPD.