DAS(28) in depth

The design of the DAS dates back to 1983. At that time a modification of an existing disease activity index was used to evaluate two gold compounds in a rather small clinical trial (1). In 1985 we started to assess all patients with an early rheumatoid arthritis (RA) regularly at our outpatient department. In this huge dataset we evaluated all the clinical and laboratory variables for their validity to assess disease activity. As a golden standard for disease activity was lacking, patients were classified as having high or low disease activity based on a joint decision of the rheumatologist and the patient. Next to this it was investigated which variables and in particular which combination of variables discriminated best between these two disease states. This work resulted in the Disease Activity Score (DAS) (2,3).

The original DAS included the Ritchie articular index, the 44 swollen joint count, the Erythrocyte Sedimentation Rate and a general health assessment on a VAS. After validation of the 28 non-graded joint count for tenderness and swelling, we developed a DAS28 including these 28 joint counts. The results of the DAS and the DAS28 are not directly interchangeable, as the DAS has a range varying from 1 up to 9 and the DAS28 ranges from 2 up to 10. Therefore, a transformation formula has been developed with which one can calculate the DAS28 from the DAS: DAS28= (1,072 x DAS) + 0,938. Serial measurements of the DAS and DAS28 are strong predictors of physical disability and radiological progression (4), and both indices are sensitive discriminators between patients with high and low disease activity, as well as between active and placebo treated patient groups (5).

Based on the DAS, response criteria have been developed: the EULAR response criteria. The EULAR response criteria include not only change in disease activity but also current disease activity. To be classified as responders, patients should have both a significant change in DAS and low current disease activity. Three categories are defined: good, moderate, and non-responders. Furthermore, response criteria using DAS28 were developed (6) and validated against the EULAR criteria using the original DAS and the ACR criteria, using both the comprehensive as well as the 28 joint counts (7). It turned out that the response/improvement criteria using the 28 joint counts were as valid as the criteria using the comprehensive joint counts: the discrepancy in responder status was less than 5 % between both EULAR criteria sets and the ACR criteria. We also investigated which level of the DAS corresponded with being in remission following the ARA criteria: a cut-off level of the DAS of 1.6 or a DAS28 of 2.6 corresponds with being in remission following the ARA criteria.