Responder definition of the Multiple Sclerosis Impact Scale physical impact subscale for patients with physical worsening

BACKGROUND The 29-item Multiple Sclerosis Impact Scale (MSIS-29) was developed to examine the impact of multiple sclerosis (MS) on physical and psychological functioning from a patient's perspective. OBJECTIVE To determine the responder definition (RD) of the MSIS-29 physical impact subscale (PHYS) in a group of patients with relapsing-remitting MS (RRMS) participating in a clinical trial. METHODS Data from the SELECT trial comparing daclizumab high-yield process with placebo in patients with RRMS were used. Physical function was evaluated in SELECT using three patient-reported outcomes measures and the Expanded Disability Status Scale (EDSS). Anchor- and distribution-based methods were used to identify an RD for the MSIS-29. RESULTS Results across the anchor-based approach suggested MSIS-29 PHYS RD values of 6.91 (mean), 7.14 (median) and 7.50 (mode). Distribution-based RD estimates ranged from 6.24 to 10.40. An RD of 7.50 was selected as the most appropriate threshold for physical worsening based on corresponding changes in the EDSS (primary anchor of interest). CONCLUSION These findings indicate that a ≥7.50 point worsening on the MSIS-29 PHYS is a reasonable and practical threshold for identifying patients with RRMS who have experienced a clinically significant change in the physical impact of MS.