Study to Assess Content Validity and Interrater and Intrarater Reliability of the Inclusion Body Myositis Functional Rating Scale

Background and Objectives Sporadic inclusion body myositis (IBM) is a rare, muscle-wasting disease that negatively affects health-related quality of life. Although a measure that has been developed to assess the impact of IBM, the IBM Functional Rating Scale (IBMFRS) has limited evidence of content validity or reliability, and what constitutes a meaningful change threshold; this study was conducted to address these gaps.

Methods Adult patients with a clinical diagnosis of IBM from the United Kingdom and disease area expert health care professionals from the United States and United Kingdom took part in this study. This study consisted of 5 stages including phone interviews (physicians), face-to-face interviews (patients), face-to-face ratings, phone ratings, and ratings of videos using the IBMFRS.

Results The IBMFRS adequately captures all core functional impacts of IBM, which was corroborated by both patient participants and physicians when debriefing the measure. Physicians and patient participants all thought any change on the measure would be meaningful change for a patient, either improvement or worsening. The quantitative analysis demonstrated good interrater reliability for face-to-face ratings (intraclass correlation coefficient [ICC] >0.7) and for video ratings (ICC >0.9). Intrarater reliability was excellent for face-to-face and video ratings (ICC >0.9). Equivalence between the modes of administration, face-to-face vs phone, was also excellent (ICC >0.9).

Discussion The IBMFRS is content valid in assessing the key functional impacts of IBM, and any change would be meaningful. It is reliable both within and across raters, and there is equivalence between different modes of administration (face-to-face vs phone).