People at a persistent pain service can walk it, but some struggle to talk about it: Reliability, detectable difference and clinically important difference of the six-minute walk test

Objectives: The six-minute walk test (6MWT) is a commonly used measure of functional capacity. This study is the first to investigate the test-retest reliability, minimal detectable difference (MDD) and the minimal clinically important difference (MCID) for people attending a persistent pain service. Relationships between change in 6MWT performance and change in self-reported physical, functional and psychological outcome measures were also explored. Methods: A cross-sectional repeated measures design was used with people having >9 months of pain attending an 8-week outpatient persistent pain programme. For reliability and MDD, 27 people were recruited, for MCID calculations, 32 people were recruited. The MCID was examined by dichotomising people into "improvers", or "non-improvers" based upon the Global Rating of Change (GRC) in physical abilities score. Results: The mean (SD) 6MWT distance was 389.4 (93.6) m at programme start, and 427.8 (83.0) m at week eight completion. The test-retest reliability was good (intraclass correlation coefficient = 0.89) and the MDD = 86.1 m. As there was no relationship between change in 6MWT distance and GRC physical abilities at week eight (r = 0.132, p = 0.472) the MCID could not be calculated. Furthermore, no relationships were found between change in 6MWT distance and other self-reported measures. Changes in GRC physical abilities and 6MWT were frequently discordant, with increased 6MWT for 7/11 "GRC non-improvers" and decreased 6MWT for 7/21 "GRC improvers". Conclusions: Amongst this cohort, change in physical ability may or may not be reflected by self-reported change. Objective tests of physical ability are recommended for people attending pain services, and validated tests should align with intervention aims.