Background: Physical disability is a main outcome in rheumatoid arthritis which tends to increase with comorbidities. Nevertheless it is not yet investigated to which extent comorbidities contribute to the multifactorial process of disability.
Objective: To quantify the contribution of comorbidity to physical disability in patients with rheumatoid arthritis.
Methods: In a prospective cohort study we ascertained age adjusted Charlson comorbidity index (CCIA), as well as serial measurements of disease activity and functional disability (evaluated by the Health Assessment Questionnaire Disability Index, HAQ) of 380 patients with established RA seen at our outpatient clinic over one year (June 2007 to July 2008). The association between comorbidity and physical disability was assessed via Analysis of Variance (ANOVA) and adjusted general linear regression models.
Results: We defined four patient groups with increasing levels of comorbidity (CCIA of 0, 1-2, 3-4, and 5-9; potential range: 0-38). Mean HAQ scores were significantly different across these groups (0.67, 0.80, 1.24, 1.40, respectively; p<0.001, ANOVA) also when adjusted for disease activity, gender and disease duration in the regression model (0.84, 0.88, 1.14, 1.48, respectively; p<0.001). CCIA effects on disability were similar within different strata of disease activity, namely remission (0.26, 0.31, 0.48, and 0.88, p<0.01); low disease activity (0.83, 0.78, 0.98, and 1.36, p<0.01); and moderate to high disease activity (1.22, 1.33, 1.70, and 1.91, p<0.01) and thus independent of disease activity. Several sensitivity analyses, including the use of SF-36, confirmed these observations.
Conclusion: Physical disability is worsening with increasing level of comorbidity, irrespective of disease activity.
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