Investigating the minimal important difference in ambulation in multiple sclerosis: A disconnect between performance-based and patient-reported outcomes?

Objective We sought to estimate the MID on two patient-reported outcome (PRO) measures that are frequently used in multiple sclerosis (MS) clinical research: the MS Walking Scale and the MS Impact Scale-29. We anchored the Minimally Important Differences with an objective measure of ambulation, the accelerometer.

Methods This secondary analysis used longitudinal data from an observational study of symptoms and physical activity in 269 people with Relapsing-Remitting Multiple Sclerosis. Participants completed a battery of PRO questionnaires, and then wore an accelerometer for seven days at each data collection time point every six months for 2.5 years. Statistical analysis first defined Change Groups on the basis of the performance-based accelerometer scores, anchored to 0.5 standard deviation change; then change was defined on the basis of published and linked MIDs for the PROs.

Results The performance-based (accelerometer) and PRO-based change distributions were stable over time. Raw scores among the accelerometer and PRO measures were associated with large effect sizes, and PRO change scores were associated with each other but not with accelerometer change scores.

Conclusions These findings contradict a central assumption that may underlie clinical research studies: that a cross-sectional correlation implies that change in PROs will correspond with change in behavior/performance. Possible explanations related to accuracy of the performance-based measure, as well as response shift effects on the PROs are discussed.