INTERPRETING SMALL DIFFERENCES IN FUNCTIONAL STATUS - THE 6-MINUTE WALK TEST IN CHRONIC LUNG-DISEASE PATIENTS

Functional status measurements ape often difficult to interpret becaus e small differences may be statistically significant but not clinicall y significant, Mow much does the Six Minute Walk test (6MW) need to di ffer to signify a noticeable difference in walking ability for patient s with chronic obstructive pulmonary disease (COPD)? We studied indivi duals with stable COPD (n = 112, mean age = Qa yr; mean FEV(1) = 975 m i) and estimated the smallest difference in 6MW distances that was ass ociated with a noticeable difference in patients' subjective compariso n ratings of their walking ability. We found that the 6MW was signific antly correlated with patients' ratings cad their walking ability rela tive to other patients (r = 0.59, 95% confidence interval [CI]: 0.54 t o 0.63). Distances needed to differ by 54 m for the average patient to stop rating themselves as ''about the same'' and start rating themsel ves as either ''a little bit better'' or ''a little bit worse'' (95% C f: 37 to 71 m). We suggest that differences in functional status can b e statistically significant but below the threshold at which patients notice a difference in themselves relative to others; an awareness of the smallest difference in walking distance that is noticeable to pati ents may help clinicians interpret the effectiveness of symptomatic tr eatments for COPD.