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.