Validation of disease activity indices using the 28 joint counts in systemic sclerosis

To validate the Disease Activity Score 28 using ESR (DAS28-ESR) and CRP (DAS28-CRP), the Simplified Disease Activity Index and the Clinical Disease Activity Index used in RA for SSc patients.

Seventy-seven SSc patients, 40 RA patients, 20 patients with primary RP (PRP) and 28 healthy volunteers were assessed. Besides the disease activity composite indices, the European Scleroderma Study Group Activity Index (EScSG-AI), the HAQ-DI, the Cochin Hand Function Scale and the Short Form Health Survey (SF36) were evaluated. The validation procedure included the assessment for truth, discrimination and feasibility.

DAS28-ESR, DAS28-CRP, Simplified Disease Activity Index and Clinical Disease Activity Index showed significant correlation with EScSG-AI, HAQ-DI, Cochin Hand Function Scale and the physical component of SF36 (P < 0.001). All four indices discriminated patients with SSc from RA, PRS and healthy controls, respectively (P < 0.01). With the exception of DAS28-CRP, the other three indices also discriminated between subgroups of SSc based on value of EScSG-AI (⩽3 and >3) (P < 0.05). All four disease activity composite indices showed a good inter- and intraobserver reliability based on repeated measures of two independent investigators (P < 0.001).

All four disease activity composite indices were found to be valid measures for assessing arthritis in SSc. DAS28-ESR showed the best performance regarding reliability and construct validity.