To study the course of impairment measured by Signals of Functional Impairment (SOFI) and disability measured by the Health Assessment Questionnaire (HAQ) over 20 years in rheumatoid arthritis (RA) patients followed from the time of diagnosis, and to explore the contribution of disease activity, joint damage, and comorbidity to variation of SOFI and HAQ over time.
Patients diagnosed with RA from 1985-1989 were prospectively monitored. There were 183 patients, 63% women, mean ± SD age 52 ± 12 years. Disease activity was measured by the 44-joint Disease Activity Score (DAS), joint damage by Larsen score, and comorbidity by the Charlson Comorbidity Index. The contribution of comorbidity, DAS, and joint damage on development of SOFI and HAQ was studied at 0, 5, 10, 15, and 20 years followup (hierarchical regression model) and over the total study period using a longitudinal regression model.
SOFI progressed over 20 years while progression of HAQ levelled off after 10 years. For SOFI, DAS and joint damage contributed the most (2-28% and 3-31%, respectively). Over 20 years, SOFI was explained by DAS (20%), joint damage (20%), age (7%), and comorbidity (4%). For HAQ, DAS contributed the most (4-24%). Over 20 years, HAQ was explained by DAS (20%), joint damage (2%), sex (7%), comorbidity (6%), and age (4%).
Over 20 years, 51% of the variation of SOFI and 39% of the variation of HAQ could be explained by age, sex, variations in comorbidity, disease activity, and joint damage. Over time, disease activity contributed significantly to both SOFI and HAQ. Joint damage contributed predominantly to SOFI.