Evaluation of reliability, validity, and responsiveness of the CDASI and the CAT-BM.

To properly evaluate therapies for cutaneous dermatomyositis (DM), it is essential to administer an outcome instrument that is reliable, valid, and responsive to clinical change, particularly when measuring disease activity. The purpose of this study was to compare two skin severity DM outcome measures, the Cutaneous Disease and Activity Severity Index (CDASI) and the Cutaneous Assessment Tool-Binary Method (CAT-BM), with the Physician Global Assessment (PGA) as the "gold standard". Ten dermatologists evaluated 14 patients with DM using the CDASI, CAT-BM, and PGA scales. Inter- and intra-rater reliability, validity, responsiveness, and completion time were compared for each outcome instrument. Responsiveness was assessed from a different study population, where one physician evaluated 35 patients with 110 visits. The CDASI was found to have a higher inter- and intra-rater reliability. Regarding construct validity, both the CDASI and the CAT-BM were significant predictors of the PGA scales. The CDASI had the best responsiveness among the three outcome instruments examined. The CDASI had a statistically longer completion time than the CAT-BM by about 1.5 minutes. The small patient population may limit the external validity of the findings observed. The CDASI is a better clinical tool to assess skin severity in DM.