Objective: To study whether the Disease Activity Score (DAS) is a valid measure of disease activity in undifferentiated arthritis (UA).
Methods: Data from a randomized, double-blind, placebo-controlled trial of methotrexate (MTX) and placebo involving 110 patients with UA were used. Data included baseline and 3, 6, 9, and 12 months, as well as diagnosis at 18 months. Validity of the DAS was analyzed using factor analysis, correlations with disease activity variables, correlations with changes in disability and joint damage, differences in DAS between diagnoses, and detecting the difference between placebo and MTX.
Results: Three disease activity factors were retrieved from the disease activity variables: patient reported outcomes, tender and swollen joints, and acute phase reactants. The DAS had its highest correlations (r > 0.77) with tender joint counts, followed by swollen joint counts (r > 0.63) and patient reported outcomes (r > 0.30), but the DAS correlated less with C-reactive protein levels (r = 0.32). Over time, the DAS was related to the Health Assessment Questionnaire response with an odds ratio of 4.1 (95% confidence interval 2.1-8.0), but not with change in joint damage. At 18 months, the mean DAS was 2.6 for rheumatoid arthritis patients, 2.2 for UA patients, and 1.9 for patients in remission (P = 0.001). The DAS discriminated better than all single variables between MTX and placebo, with a Guyatt's effect size of 0.89.
Conclusion: The DAS appears to be a reasonably valid measure of disease activity for use in UA clinical trials.