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

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 < 0.0001, analysis of variance; discriminant validity). All scores exhibited similar correlations with radiological progression (construct validity) over 3 years (R = 0.54–0.58; P < 0.0001).APRs add little information on top (and independent) of the combination of clinical variables included in the SDAI. A purely clinical score is a valid measure of disease activity and will have its greatest merits in clinical practice rather than research, where APRs are usually always available. The CDAI may facilitate immediate and consistent treatment decisions and help to improve patient outcomes in the longer term.Rheumatoid arthritis (RA) is a progressive inflammatory disease, which causes damage and disability [1-5] that can be prevented by promptly initiated and effective therapy [6-9]. To ensure that therapy is effective, frequent clinical assessments are needed [10-12]. For the purpose of disease activity assessment, valid assessment tools using the well established ACR/EULAR/WHO–ILAR (American College of Rheumatology/European League Against Rheumatism/W