Submaximal exercise testing in the assessment of interstitial lung disease secondary to systemic sclerosis: reproducibility and correlations of the 6-min walk test

Background: The 6-min walk test (6MWT) is increasingly used as an outcome measure in interstitial lung disease (ILD). Aim: To evaluate the usefulness of the 6MWT in a cohort of patients with ILD secondary to systemic sclerosis (SSc) and to correlate with established physiological parameters. Methods: 163 patients with SSc-ILD were recruited for a multicentre, randomised, double-blind clinical trial. Available data at protocol screening included repeated 6MWTs, pulmonary function testing with diffusing capacity, Doppler echocardiography and high-resolution computed tomography of the thorax. Borg Dyspnoea Index was evaluated before and after 6MWT. Results: Mean ( standard deviation (SD)) distance walked during walk test 1 was 396.6 (84.55) m compared with 399.5 (86.28) m at walk test 2. The within-subject, intertest correlation as determined by Pearson's correlation coefficient testing was 0.95 (p < 0.001). However, only weak correlations of 6MWT with percentage forced vital capacity and the Borg Dyspnoea Index were observed, and no correlation was observed with percentage diffusing capacity. Conclusion: These data confirm the high reproducibility of the 6MWT in patients with SSc-ILD and therefore the validity of the test in this cohort. The lack of correlation of 6MWT with standard physiological parameters of ILD suggests a multifactorial basis for limited exercise capacity in patients with SSc and calls into question the utility of the 6MWT as a measure of outcome in future studies on SSc-ILD.