Increase in walk distance is not enough evidence to add a walk to the 6-minute walk test

To the Editors:

We read with interest the work of Hernandes et al. [1]. The authors performed a cross-sectional analysis using data from 1,514 patients with moderate-to-very severe chronic obstructive pulmonary disease who performed 6-min walks on consecutive days and recommended that, because the distance of the second walk was longer than that of the first, two walks should be performed [1]. Their finding that the walk distance increases by 7% on the second walk (27-m increase from a baseline of 391 m) validates our previous findings and adds new knowledge about the variables that predict improvement in walk distance [2]. These predictors of improvement may be helpful in identifying which subgroups of patients may benefit the most from performing two walks instead of one per 6-min walk test.

However, documenting an increase in walk distance is not enough evidence to add an additional walk to the 6-min walk test. To determine whether an additional walk should be performed, the authors should investigate whether adding a second walk makes the test more accurate, i.e. allows better correlation of changes in walk distance with a gold-standard measurement of exercise capacity, such as a cardiopulmonary exercise testing, or a better representation of change in patient-reported activity limitation measured by symptom-based questionnaires [3]. Also, it should be determined which walk distance leads to more precise test results over longitudinal follow-up, i.e. has less random error reflected by a smaller standard deviation. Less random error would lead to reduced sample size requirements for clinical trials that use improvement in walk distance as the end-point, which will make these studies much more feasible. If the second walk were to appear superior in these analyses, the magnitude of the benefit would have to be weighed against the added cost and effort required for a second walk. Unfortunately, the authors do not include a gold-standard measurement of exercise capacity, patient-reported exercise limitation, longitudinal follow-up or data on relative magnitude of benefit, and therefore, do not provide enough evidence to support their recommendation to add a second walk.

Even if an additional walk were to be performed, should we be using the second walk distance as the formal result of the test in all our patients, as implied by the authors [1]? 18% of their patients walked a shorter distance on the second walk. This could have occurred for a number of reasons, such as residual fatigue from the first walk or a lack of motivation to repeat the walk distance. In such cases, this shorter second walk distance may not best represent true exercise capacity; rather, the best of two, or even an average of two, walk distances may be a better alternative. However, Hernandes et al. [1] did not determine the optimal interpretation of the two walk distances.

To support their recommendation for using the second walk distance, the authors make indirect arguments in their discussion based on a Bland–Altman analysis that compared the first walk distance to the second. However, the Bland–Altman analysis is a test of agreement, and cannot be used to determine which measurement is better [4].

Therefore, noticing an increase in the 6-min walk distance and identifying factors that predict the increase is a useful addition to our understanding of the 6-min walk test, but does not provide enough evidence to recommend an additional walk be performed. Unless such evidence becomes available, the benefit of a second 6-min walk remains speculative [5].

• ©ERS 2011