The idiopathic inflammatory myopathies (IIM) are multisystem, autoimmune diseases with many different clinical presentations and subtypes and predominantly affecting muscles [1]. Subtypes include dermatomyositis, polymyositis, immune-mediated necrotizing myopathy, inclusion body myositis, overlap myositis, and antisynthetase syndrome [2,3]. Muscle weakness and impaired physical function are usually a hallmark feature of IIM, but extramuscular manifestations can be present including skin rash, interstitial lung disease, cardiac dysfunction, and arthritis [4].

Measuring patient-reported response to life impacts in clinical trials has been challenging as there are no fully validated patient reported outcome measures (PROs) that exist for adult patients with IIM. The Outcome Measures in Rheumatology (OMERACT) Myositis Working Group has been evaluating the validity of candidate life-impact instruments to suit the needs of people living with, caring for, and researching IIM for nearly a decade [5]. To date, the Health Assessment Questionnaire-Disability Index (HAQ-DI) has been substantially used in IIM clinical trials and cohorts to measure physical function but does have limitations [6], including ceiling and floor effects, lack of content validity, and limited responsiveness. These limitations prompted further exploration by the OMERACT Myositis Working Group to find suitable instruments to measure physical function among other domains [7].

Through exhaustive literature reviews [8], international patient focus groups [9], modified Delphi surveys [10], and in-person workshops, the core domains of pain interference, fatigue, and physical function were identified by the OMERACT Myositis Working Group. The content validity of candidate instruments was then evaluated through cognitive interviewing [11]. The Patient Reported Outcomes Measurement Information System (PROMIS) pain interference 6a, physical function 8b, and fatigue 7a instruments emerged as candidate instruments through this process. Further validation revealed excellent test-retest reliability and construct validity [12].

The purpose of this study was to determine the responsiveness and minimal important difference (MID) for the proposed PROMIS candidate instruments (pain interference 6a, physical function 8b, and fatigue 7a) in adults with IIM.