Objective: To assess the factorial structure of the Disease Activity Score including a 28-joint count (DAS28) if applied in patients with rheumatoid arthritis (RA) and psoriatic arthritis (PsA).
Methods: DAS28 values from 85 consecutive PsA outpatients and 2 RA patient cohorts comprising 85 patients each were compared. The first RA cohort (RA1) consisted of age- and sex-matched patients seen during the same period as the patients with PsA. The first 85 RA outpatients from September 2003 were included in the second cohort (RA2). Item weighting, factor loading, and internal consistency were assessed by factor analysis, principal component analysis, and calculation of Cronbach's alpha.
Results: The mean +/- SD DAS28 scores of patients in the PsA, RA1, and RA2 cohorts were 3.2 +/- 1.31, 3.21 +/- 1.45, and 3.79 +/- 1.44, respectively. A significant difference between the PsA and RA2 cohorts was found for DAS28 (P = 0.0063), swollen joint count (P = 0.007), and patient's global assessment (P < 0.001), but not for erythrocyte sedimentation rate. Internal consistency of the DAS28 in patients with PsA was considerably lower, item weighting showed remarkable differences, and factor analysis revealed that the DAS28 constitutes a bidimensional instrument in patients with PsA, whereas in both RA cohorts it appeared to be monodimensional.
Conclusion: With respect to its statistical properties, the DAS28 proved to be considerably different in PsA compared with RA. Therefore its application for disease activity assessment in patients with PsA cannot be recommended without a formal validation procedure.