Six-minute treadmill distance underestimates six-minute walk distance in severely limited patients.

Lenssen et al. outlined in their recent paper that the six-minute walk test (6-MWT) performed on a treadmill or in a hallway are not interchangeable because of a large between-test variation. However, they claimed that there is no general bias between the two procedures to measure the six-minute walk distance (6-MWD). This might be true for the total study group as outlined in their histogram of the differences. However, a closer look at the Bland-Altman plot reveals that the differences between the tests depend on the walking distance itself, that is the more limited patients have indeed shorter treadmill results, whereas the bulk of patients with a fairly normal 6-MWD of 500–700m had a slightly better treadmill result. When you consider this effect, the results are no longer in contrast to the randomized study of Stevens et al.. They investigated patients from a pulmonary rehabilitation program with a mean 6-MWD of 374 78m (1228 255 feet) and found a systematic underestimation of the treadmill distance. So, inexperienced patients with limited exercise capacity probably fear to walk on a treadmill, whereas subjects with an almost normal exercise capacity might be stimulated by the treadmill setting. The six-minute walk test has shown to be of greater prognostic value if the patient is severely limited and the test resembles almost a symptom-limited exercise test. Therefore, the test on the treadmill should be omitted in those patients where the 6-MWT is most interesting. I even suggest not talking about a 6-MWT when it is a ‘six-minute treadmill test’. The term ‘6-MWT’ should be reserved for those tests that strictly abide to the guidelines of the American Thoracic Society.