Background/Purpose: To study the contribution of disease activity, joint damage and co-morbidity on development of impairment measured by signals of functional impairment (SOFI) and disability measured by health assessment questionnaire (HAQ) in rheumatoid arthritis (RA) patients prospectively followed over 20 years after diagnosis.

Methods: 183 RA patients diagnosed between 1985 and 1989 were prospectively monitored over 20 years. There were 116 (63 %) women, mean (SD) age was 52 (12) years and symptom duration before inclusion was 11 (7) months. Disease activity was measured by 44-joint DAS, joint damage by Larsen score of radiographs of hands and feet, comorbidity by Charlson Comorbidity Index (17 diagnoses each weighted by mortality risk), impairment by SOFI (3-parts performance based index measuring hand, arm and leg function) and disability by HAQ. Two separate multiple regression models with SOFI and HAQ as outcome variables at 0, 5, 10, 15 and 20 year follow up were created.

Results: Altogether, disease activity, radiographic joint damage and co-morbidity explained 22-38% of SOFI and 14-38 % of HAQ (figure). For SOFI, DAS contributed with 2 -27 % with a peak at 5 years. Radiographic damage contributed increasingly (6-35%). For HAQ, DAS contributed significantly at all follow up times (7-28%), with a peak at 5 years whereas radiographic damage had minor contribution (0-10%). Comorbidity showed minor contribution both to SOFI and HAQ.

Conclusion: In the long-term perspective impairment is increasingly explained by radiographic damage, whereas disability is less well explained by RA related factors over time. Comorbidity contributed only to small extend to both impairment and disability.

Figure. Contribution of disease activity (DAS), joint damage (Larsen score) and comorbidity (Charlson comorbidity index) to impairment (SOFI) and disability (HAQ) in rheumatoid arthritis patients over 20 years