title: Validity of the disease activity score in undifferentiated arthritis.
abstract: 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.
  
  
  CONCLUSIONS
  The DAS appears to be a reasonably valid measure of disease activity for use in UA clinical trials.
authors.affiliation: Radboud University Nijmegen Medical Centre, The Netherlands.
authors.forename: Jaap
authors.initials: J
authors.lastname: Fransen
authors.forename: Karen
authors.initials: K
authors.lastname: Visser
authors.forename: Henrike
authors.initials: H
authors.lastname: van Dongen
authors.forename: Tom
authors.initials: T
authors.lastname: Huizinga
authors.forename: Piet L C M
authors.initials: PL
authors.lastname: van Riel
authors.forename: Désirée M
authors.initials: DM
authors.lastname: van der Heijde
mesh_terms: D000328:Adult; D000368:Aged; D001168:Arthritis / Q000175:diagnosis* / Q000188:drug therapy / Q000473:pathology*; D001172:Arthritis, Rheumatoid / Q000175:diagnosis / Q000188:drug therapy / Q000473:pathology; D018450:Disease Progression; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D008297:Male; D008727:Methotrexate / Q000627:therapeutic use; D008875:Middle Aged; D012720:Severity of Illness Index
chemical_list: D008727:Methotrexate