Validity and responsiveness of the Work Functioning Impairment Scale (WFun) in rheumatoid arthritis patients: A multicenter prospective study.

Previous studies have not fully evaluated the measurement properties of the Work Functioning Impairment Scale (WFun) in clinical situations and there are no data from patients with rheumatoid arthritis (RA). This study aimed to investigate the convergent validity and responsiveness of the WFun in patients with RA. This substudy was a part of the multicenter, observational, prospective FIRST ACT-SC study. In total, 322 paid workers with RA received anti-rheumatic drugs at baseline. The WFun, Disease Activity Score based on 28 joints and erythrocyte sedimentation rate (DAS28-ESR), Clinical Disease Activity Index (CDAI), Simplified Disease Activity Index (SDAI), Work Productivity and Activity Impairment Questionnaire (WPAI), and Japanese Health Assessment Questionnaire Disability Index (HAQ-DI) were administrated at baseline and weeks 12, 24, and 52. Multilevel regression analyses were performed. High average WFun scores were associated with higher score categories of the DAS28-ESR, CDAI, SDAI, WPAI and HAQ-DI. Average WFun change scores linearly decreased with decreases on each instruments. The findings indicate that the WFun has reasonable measurement properties: the scale demonstrated validity and responsiveness with RA patients. The WFun could be useful in assessing presenteeism in patients with RA.