How much disability in rheumatoid arthritis is explained by rheumatoid arthritis?

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Abstract
Objective To measure the proportion of disability explained by disease manifestations compared with nondisease factors in rheumatoid arthritis (RA). Methods A hypothetical model of the disablement process specific for RA was constructed using the demographic, sociocultural, and clinical characteristics of a consecutive cohort of RA patients. Disability was measured with the modified Health Assessment Questionnaire (M-HAQ) and the physical function scale of the Medical Outcomes Study Short Form 36 (SF-36) questionnaire. Independent variables, grouped according to their position in the RA disablement process model, were sequentially entered in a series of hierarchical regression models. The proportion of variance in disability explained by each group of variables was measured by the group's incremental R2. Results The overall proportion of disability explained by the full model was 59%. Factors in the main disease–disability pathway explained 33%, of which 3% was explained by disease duration, 5% by the Westergren erythrocyte sedimentation rate, 14% by articular signs and symptoms, and 11% by performance-based functional limitations. External modifiers and contextual variables explained 26% of the variance in disability, of which age and sex accounted for 2%, formal education 4%, psychological status 17%, and symptoms of depression 3%. Conclusion Both the main disease–disability pathway and factors external to this pathway contribute significantly to disability in RA. These findings provide evidence of the relative influence of psychosocial factors, compared with disease manifestations, on the disability of patients with RA.