• Clinical Epidemiology
• COPD epidemiology
• Exercise
• Pulmonary Rehabilitation

We thank the co-chairs of the joint American Thoracic Society (ATS) and the European Respiratory Society (ERS) task force for their comments on our paper and interest in our article on reference values for the 6-min walk test (6MWT) performed over a 10m course. In their correspondence, the shorter course length is considered as one specific protocol deviation from the ATS statement1 that does not represent a conventional 6MWT. What is the ‘conventional’ 6MWT? Fourteen years ago, the ATS guidelines were published with the advice to test on a 30m course. However, at that time some studies used other course lengths, as was mentioned in the statement.1 Since then, more studies2–4 and healthcare providers have used a shorter course length due to space limitations. Moreover, 14 reference equations have been established for the 6MWT since 2002, of which 6 are for course lengths other than 30 m, ranging from 10 m to 45 m.5 ,6 Obviously, it is incorrect to only rename the 6MWT over a 10 m course as 10 m 6MWT (as the writers purpose). In that case, we suggest to rename all variants (20 m 6MWT, 45 m 6MWT, 50 m 6MWT) and to refer to specific reference values obtained at similar course length. We do militate for a clear methodological description of the 6MWT, including course length.

Other tests, such as the incremental shuttle walk or the 4 m gait serve different goals then the 6MWT,7 ,8 whereas the 6MWT is a very practical comparison with walking in everyday life in contrast with the incremental shuttle walk (a non-self-paced test).7 The 6MWT is a sensitive indicator of clinical change in most cardiopulmonary conditions, especially in COPD where it demonstrates functional responses with improvement of the 6-min walk distance (6MWD).9 ,10 The other tests amplify the test battery but cannot fully replace the 6MWT as a functional capacity test.8 The perceived need for the 10 m reference equations in everyday practice was confirmed by the many requests for the norm values we received after the article was published.

The test-retest reliability for the 6MWD over 10 m has an intraclass correlation coefficient (ICC) consistency of 0.98 (95% CI 0.96 to 0.99 and 95% of the difference scores within the limits of agreement: −42.33 to 41.56 m).5 The number of subjects needed to achieve reliable prediction models were used and the models appeared to be reliable without undue influence of any subset of cases (the article's online supplement).6 The test-retest reliability in patients with COPD was very high (ICC=0.98) and consistent with previous studies.5 As was mentioned in our previous article, future research is needed to study the validity and responsiveness of the 6MWT over a 10 m course.5

Whether absolute ‘benchmark values’, established in research using 30 m or larger courses, are suitable for a test conducted over 10 m is indeed not clear yet, neither is it for a test over 50 m (on which reference equations were conducted by Troosters et al 11), and should be studied. Relative benchmark values, such as achieving a 6MWT distance of less than 82% of the predicted value considering abnormal,11 still apply.

In accordance to Singh et al, we encourage researchers and clinicians to use published reference equations for the 6MWT related to the length of the test course.6 Moreover, an update of the ATS guidelines is timely. New literature was published since 2002 and there is a need for adaptations of functional exercise tests in different clinical settings, especially in primary care.