Evaluation of the appropriateness of composite disease activity measures for assessment of psoriatic arthritis

Objective Specific composite indices assessing disease activity in psoriatic arthritis (PsA) have not yet been sufficiently validated. The objective of this study was to identify instruments best reflecting disease activity in PsA. Methods Measures for inclusion in clinical trials, as recommended by the OMERACT-7/8 PsA workshops, were assessed. A principal component analysis (PCA) was performed with cross-sectional data of 105 patients with PsA to identify a minimal set of important dimensions for a disease activity assessment tool for PsA. This was compared with components contained in existing composite indices. Results The PCA revealed four principal components best reflecting disease activity. The first contained patient global and pain assessment; the second contained 66/68 swollen and tender joint counts as main variables; C-reactive protein (CRP) best loaded to the third component; and the fourth was loaded by skin assessment but did not reach significance. When comparing the three significant principal components with items of established composite measures, they best covered by the Disease Activity Index for Reactive Arthritis (DAREA) which comprises patient pain and global assessments, 66/68 joint counts and CRP. Conclusion Among the currently available indices used in arthritic conditions, the DAREA best reflects the domains found to be important in PsA.