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

Objective: 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.

Methods: 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.

Results: 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 (r = 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.

Discussion: 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.