Test-retest reliability, agreement, and minimal detectable change in the 6-minute walk test in patients with intermittent claudication

Abstract

ObjectiveStandardized walk tests are important for objective assessment of walking distance in patients with intermittent claudication (IC). The 6-minute walk test (6MWT) has been suggested to correlate more closely than testing on a treadmill with everyday ambulatory function, but its measurement properties have hardly been studied in IC. The aim of this study was to determine the test-retest reliability, agreement, standard error of measurement (SEM), and minimal detectable change of the 6MWT in patients with IC. MethodsThis reliability and agreement study recruited 102 patients with stable IC (mean age, 72 ± 7.4 years; 43 women) from the vascular surgery outpatient clinic at Sahlgrenska University Hospital in Sweden. The patients performed the 6MWT twice, with at least 30 minutes of rest between tests. To determine test-retest reliability, the intraclass correlation coefficient was calculated. Bland-Altman plots were used to measure agreement. ResultsThe mean walking distance in both test and retest was 397.8 m (standard deviation, 81.2 m; N = 100), and the individual walking distance varied from 175 to 600 m. Excellent test-retest reliability for the 6MWT (intraclass correlation coefficient, 0.95; 95% confidence interval, 0.9-0.97) was observed. The SEM was 16.6 m (95% confidence interval, 14.6-19.3), the SEM percentage was 4.2%, and the minimal detectable change was 46 m. Five observations (5%) were positioned outside the limits of agreement; there was a small proportional bias, and the scatter of values for differences decreased as the average values increased. ConclusionsThe excellent test-retest reliability implies that it is sufficient for a patient with IC to perform the 6MWT once, at every test occasion. For the individual, an improvement or deterioration in maximum walking distance of >46 m after an intervention would be required to be 95% confident that the change is significant. Being a simple and clinically useful test, the 6MWT can be widely used to evaluate the effects of different interventions in patients with IC.