Acute phase reactants add little to composite disease activity indices for rheumatoid arthritis: validation of a clinical activity score

Frequent assessments of rheumatoid arthritis (RA) disease activity allow timely adaptation of therapy, which is essential in preventing disease progression. However, values of acute phase reactants (APRs) are needed to calculate current composite activity indices, such as the Disease Activity Score (DAS)28, the DAS28-CRP (i.e. the DAS28 using C-reactive protein instead of erythrocyte sedimentation rate) and the Simplified Disease Activity Index (SDAI). We hypothesized that APRs make limited contribution to the SDAI, and that an SDAI-modification eliminating APRs - termed the Clinical Disease Activity Index (CDAI; i.e. the sum of tender and swollen joint counts [28 joints] and patient and physician global assessments [in cm]) - would have comparable validity in clinical cohorts. METHOD: Data sources comprised an observational cohort of 767 RA patients (average disease duration 8.1 +/- 10.6 years), and an independent inception cohort of 106 patients (disease duration 11.5 +/- 12.5 weeks) who were followed prospectively.

Our clinically based hypothesis was statistically supported: APRs accounted only for 15% of the DAS28, and for 5% of the SDAI and the DAS28-CRP. In both cohorts the CDAI correlated strongly with DAS28 (R = 0.89-0.90) and comparably to the correlation of SDAI with DAS28 (R = 0.90-0.91). In additional analyses, the CDAI when compared to the SDAI and the DAS28 agreed with a weighted kappa of 0.70 and 0.79, respectively, and comparably to the agreement between DAS28 and DAS28-CRP. All three scores correlated similarly with Health Assessment Questionnaire (HAQ) scores (R = 0.45-0.47). The average changes in all scores were greater in patients with better American College of Rheumatology response (P