Aim: The aims of this study were to determine the reliability, responsiveness and minimally important change score of the Multiple Sclerosis Impact Scale (MSIS)-29 physical using the Expanded Disability Status Scale (EDSS) as an anchor measure.
Methods: 214 patients with multiple sclerosis (MS) (EDSS 0-8.5) had concurrent MSIS-29 and EDSS assessments at baseline and at up to 4 years of follow-up.
Results: 116 patients had unchanged EDSS scores. Stability of the MSIS-29 physical (mean change 0.1 points) was better in the 85 patients with EDSS 0-5.0 than in the 31 patients with EDSS 5.5-8.5 in whom the MSIS-29 physical score fell by 8 points, a response shift phenomenon. A floor effect for the MSIS-29 was observed in 5% of stable patients at both time points. 98 patients experienced EDSS change with moderately strong statistically significant correlations between change scores in the EDSS and the MSIS-29 physical (r = 0.523, p<0.0001). Effect sizes for MSIS-29 physical change were moderate to large. Using receiver operating characteristic curves, the MSIS-29 change score which produced a combination of optimal sensitivity and specificity was chosen for both EDSS ranges. For EDSS range 5.5-8, a change score of 8 had a sensitivity of 87% and specificity of 67%. For EDSS 0-5.0, a change score of 7 had a sensitivity of 78% and a specificity of 51%.
Conclusions: The MSIS-29 physical performs well over time, and is suitable for use in trials; a minimal change score of 8 points in the MSIS-29 is clinically significant.