Minimal clinically important difference for improvement in six-minute walk test in persons with knee osteoarthritis after total knee arthroplasty

BACKGROUND: The interpretability of the six-minute walk test (6MWT) in individuals with knee osteoarthritis (OA) is unclear. We aimed to determine the minimal clinically important difference (MCID) for improvement in 6MWT in persons with knee OA at 12 months after total knee arthroplasty (TKA), and if it differed by baseline walking ability. METHODS: Participants with knee OA were assessed 1 month pre- and 12 months post-TKA, including completion of 6MWT. At 12 months, participant-perceived change in walking ability was assessed on an 8-point Likert scale ranging from “extremely worse” to “extremely better”. Using logistic regression, ROC curves examined the ability of change in 6MWT distance to discriminate those who perceived walking was improved. MCID was selected overall and then by quartile of baseline 6MWT distance using the Youden method. RESULTS: Two hundred seventy-eight participants were included: mean age 67 years (SD 8.5), 65.5% female, mean pre-TKA 6MWT distance 323.1 (SD 104.7) m, and mean 12-mo 6MWT distance 396.0 (SD 111.9) m. The overall MCID was 74.3 m (AUC 0.65). Acceptable model discrimination (AUC > 0.70) was achieved for individuals in the lowest quartiles of baseline 6MWT distance: Quartile 1: MCID 88.63 m (AUC 0.73); Quartile 2: MCID 84.47 m (AUC 0.72). CONCLUSIONS: In persons with knee OA 12 months post-TKA, 6MWT MCID is dependent on baseline walking ability. Poor model discrimination for those in the highest (best) quartiles of baseline walking ability raise questions about 6MWT use across the full spectrum of walking ability. Further research is needed to better understand use of 6MWT as a performance-based measure of physical function for persons with knee OA.