International Journal of COPD (Oct 2017)
Minimal important difference and responsiveness of 2-minute walk test performance in people with COPD undergoing pulmonary rehabilitation
Abstract
Kylie Nicole Johnston, Adrian James Potter, Anna Caroline Phillips School of Health Sciences, Sansom Institute for Health Research, University of South Australia, Adelaide, SA, Australia Background and objectives: Field exercise tests (eg, 6-minute walk test [6MWT]) are important measures of functional exercise capacity in people with COPD. Shorter tests such as the 2-minute walk test (2MWT) may offer advantages in some populations but lack information about responsiveness to change. This study examined responsiveness, minimal important difference (MID), test–retest reliability, and construct validity of the 2MWT in people with stable COPD attending outpatient pulmonary rehabilitation (PR).Methods: At pre-PR assessment, study participants completed a 2MWT twice in addition to usual measures (6MWT and Chronic Respiratory Questionnaire). At post-PR assessment following a standard PR program, measures were repeated and global rating of change scores obtained (patient and therapist). Pre–post program change scores were examined for correlations with change in 2-minute walk distance (2MWD) and used (where r≥0.3) to estimate the MID through anchor-based methods. Distribution-based estimates based on standard error of measurement were examined. Test–retest reliability (intraclass correlation coefficient [ICC] and Bland–Altman agreement) and validity (Pearson correlation with 6-minute walk distance [6MWD]) were reported.Results: Fifty-nine people (28 men) with stable COPD, mean age 68 years (SD 10 years), and percentage predicted forced expiratory volume in 1 second 48% (SD 20%) attended pre-PR assessment. Test–retest ICC for same-session 2MWD was 0.985. A mean difference of 2.4 m (95% confidence interval [CI] 0.7–4.0 m, P=0.006) occurred between the first and second trials. 2MWD and 6MWD were highly correlated (r=0.87, P<0.001). Forty-one participants completed PR and were included in responsiveness and MID analysis. Mean 2MWD improved significantly post-PR (8.8 m, 95% CI 3.6–14 m, P=0.001). The MID in 2MWD, anchored against clinically meaningful change in 6MWD, was 5.5 m (area under curve =0.81, P=0.001). Distribution-based methods estimated an MID of 4 m.Conclusion: Change in 2MWD of at least 5.5 m following a PR program corresponded to a clinically meaningful change. A practice test is recommended due to learning effects.Keywords: exercise test, psychometrics, COPD, rehabilitation
