Psychometric testing of the Rheumatoid Arthritis Work Instability Scale in employed people with fibromyalgia

The aim of the study was to establish whether the Rheumatoid Arthritis Work Instability Scale (RA-WIS), in its current form, is applicable for use with employed people with fibromyalgia (FM) to identify the risk of work disability and need for work rehabilitation.Content validity was first investigated using cognitive debriefing interviews. Participants completed a postal questionnaire. Construct validity was assessed using Rasch analysis. Concurrent validity included testing between the RA-WIS and work (e.g., Workplace Activity Limitations Scale) and health (FM Impact Questionnaire-Revised (FIQ-R) scales. Two weeks later, participants were mailed a second questionnaire to measure test-retest reliability.Interviews were conducted with 13 participants with FM. All RA-WIS items were considered very or extremely relevant by almost all participants, with only one suggesting other items (anxiety and brain fog). Questionnaire responses were analysed from 156 employed participants: 94% women; 45.71 (SD 10.05) years of age; with time since FM diagnosis 2.99 (4.17) years (symptom duration 8.36 (SD 7.16) years). The RA-WIS mostly satisfied Rasch model requirements and a Rasch transformation scale was created. Concurrent validity was generally good (rs = 0.55-0.66) with work scales and the FIQ-R. Internal consistency (Person Separation Index values) was consistent with group use in FM, not individual level use. Test-retest reliability was excellent, with intraclass coefficient (2, 1) = 0.90.The RA-WIS is valid and reliable for group use in employed people with FM. However, further work is needed to develop a WIS for individual use in FM.