Assessing the content validity of patient-reported outcome measures in adult myositis: A report from the OMERACT myositis working group

Objective: To investigate the content validity of several patient-reported outcome measures (PROMs) in patients with idiopathic inflammatory myopathies (IIM). Methods: Seven individual PROM instruments were selected by the Outcome Measures in Rheumatology (OMERACT) Myositis Working Group relating to the following domains: pain, fatigue, physical function and physical activity. Twenty patients from the Johns Hopkins Myositis Center were selected for one-on-one face-to-face or phone interviews for cognitive interviewing of individual PROMs to assess comprehension and content validity. Additionally, patients were asked if they thought muscle symptoms, an area originally identified in qualitative studies, were encapsulated by the other four domains. Results: The majority of patients (>70%) felt that each of the instruments was clear, easy to read and understand, and could be used for assessment of its domain. Two-thirds (66%) of patients felt that ‘muscle symptoms’ were captured by the other domains. Conclusions: We provided evidence to support adequate content validity for several PROMs. Further research is needed to determine whether ‘muscle symptoms’ warrant a separate domain.