Background: To examine the longitudinal relationship between the patient-rated Multiple Sclerosis Impact Scale (MSIS-29) and the doctor-reported Multiple Sclerosis Functional Composite (MSFC).
Methods: Two-hundred and four MS patients at baseline and 150 patients one to three years later had MSFC and MSIS-29 assessments. Cross-sectional correlations between these measures and correlations of change in scores were examined. Minimally important change (MIC) in the MSFC was defined at either 0.5 or 0.32 SD from baseline. Effect sizes (ES) were calculated.
Validity: The MSIS-29 physical correlated moderately with the total MSFC score and the 25-foot timed walk and 9-hole peg test. Correlations of the MSIS-29 physical with the PASAT, and the MSFC with the MSIS-29 psychological were weak. Responsiveness: When MIC in the MSFC was defined as 0.5, mean MSIS-29 physical change was 11.26 (ES = 0.53). At MSFC change of 0.32, mean MSIS-29 physical change was 10.4 (ES = 0.52). Change in MSFC scores correlated weakly with change in the MSIS-29 scores. Stability: In patients with stable MSFC scores, the mean MSIS-29 physical scores improved minimally over time with negligible ES.
Conclusions: Although the MSIS-29 physical demonstrates moderate cross-sectional correlation with the MSFC, the weak correlations of change scores between the two instruments indicate that they measure different aspects of the effects of multiple sclerosis morbidity.