Reliability, Validity, and Responsiveness of the Patient-Specific Functional Scale for Measuring Mobility-Related Goals in People With Multiple Sclerosis

Background and Purpose: This study's purpose was to investigate the reliability, validity, and responsiveness of the Patient-Specific Functional Scale (PSFS) for measuring mobility-related goals in people with multiple sclerosis (MS). Methods: Data from 32 participants with MS who underwent 8 to 10 weeks of rehabilitation were analyzed (Expanded Disability Status Scale scores 1.0-7.0). For the PSFS, participants identified 3 mobility-related areas where they had difficulty and rated them at baseline, 10 to 14 days later (before starting intervention), and immediately after intervention. Test-retest reliability and response stability of the PSFS were calculated using the intraclass correlation coefficient (ICC2,1) and minimal detectable change (MDC95), respectively. Concurrent validity of the PSFS was determined with the 12-item Multiple Sclerosis Walking Scale (MSWS-12) and the Timed 25-Foot Walk Test (T25FW). PSFS responsiveness was determined using Cohen's d, and minimal clinically important difference (MCID) was calculated based on patient-reported improvements on a Global Rating of Change (GRoC) scale. Results: The PSFS total score demonstrated moderate reliability (ICC2,1 = 0.70, 95% CI: 0.46 to 0.84) and the MDC was 2.1 points. At baseline, the PSFS was fairly and significantly correlated with the MSWS-12 (r = -0.46, P = 0.008) but not with the T25FW. Changes in the PSFS were moderately and significantly correlated with the GRoC scale (ρ = 0.63, P < 0.001), but not with MSWS-12 or T25FW changes. The PSFS was responsive (d = 1.7), and the MCID was 2.5 points or more to identify patient-perceived improvements based on the GRoC scale (sensitivity = 0.85, specificity = 0.76). Discussion and Conclusions: This study supports the use of the PSFS as an outcome measure in people with MS to assess mobility-related goals.