6‐Minute walk distance in healthy Singaporean adults cannot be predicted using reference equations derived from Caucasian populations

We thank Dr Teramoto and colleagues for their interest in our study. Dr Teramoto and colleagues comment that we did not fully review all publications concerning reference values for the 6-min walk distance (6MWD) in Japanese individuals. Unfortunately our literature review was limited to papers in peer-reviewed journals and therefore we did not reference their Letter to the Editor1 or the Annual Report of the Japanese Respiratory Failure Research Working Group.2 We disagree with Dr Teramoto and colleagues’ assertion that the 6MWD in Asian individuals may not be very different to that in Caucasians. The basis for their assertion lies in a comparison of 6MWDs using data reported by Enright and Sherrill.3 For the following reasons we do not believe that such a comparison is valid. First, Enright and Sherrill’s3 published regression equation was derived from a single test, which will underestimate the true 6MWD as a consequence of the known effect of familiarization on 6MWD.4,5 Second, their regression equation predicts 6MWDs substantially lower than 6MWDs obtained using other equations developed in similar populations.4,5 Third, the magnitude of the heart rate response in their subjects suggests the subjects’ effort level was submaximal. Specifically, Enright and Sherrill3 report a heart rate increase of only 22 b.p.m.3 in contrast to an increase of 37 b.p.m. on the first 6MWD performed by our Singaporean subjects.6 Notably, the magnitude of the heart rate increase in both of our studies6,7 is similar to that reported by other authors who measured 6MWD in healthy adults.4 Fourth, we have recently conducted a study in healthy Caucasians7 using the identical 6MWD protocol to that used by Poh et al.6 and our regression equation derived in Caucasian subjects yields 6MWDs that exceed measured 6MWD in the Singaporean cohort by more than 100 m. For these reasons, in our paper6 we did not compare the 6MWDs in the Singaporean cohort with the 6MWDs derived using Enright and Sherrill’s regression equation.3 The 6MWD protocol we used in our healthy subjects was consistent with the guidelines recommended by the American Thoracic Society.8 However, our clinical and research experience has resulted in further refinement of this protocol by including (i) continuous measurement of heart rate throughout the test; (ii) routine use of oximetry when testing individuals with respiratory disease, because of the propensity for desaturation during the 6MWD;9,10 (iii) a familiarization walk; and (iv) standardized frequency of encouragement given to subjects who take rests during the test. We believe these additions are important for safety reasons and enable the test to be used as a valid outcome measure following interventions such as pulmonary rehabilitation. We agree with Dr Teramoto and colleagues that the 6MWD is influenced by several factors and not only by race and ethnicity. At present, most published regression equations developed in Caucasians predict 6MWD based on age, height, weight and gender and account for less than 50% of the variance in 6MWD.3,5,11 The differences in the 6MWDs predicted by these equations are most likely due to different test protocols, including different track lengths, test instructions, frequency of encouragement during the test and the number of test repetitions. We therefore agree that caution should be applied when using the available regression equations to determine an absolute value for 6MWD in an individual. In summary, we have shown that equations derived from studies of healthy Caucasians significantly overestimate 6MWD for a Singaporean Chinese population. The data provided by Dr Teramoto and colleagues1,2 suggest that such overestimation would also occur in the Japanese population.