Objective. There has been continuous debate regarding the applicability of various composite measures for the assessment of disease activity in rheumatoid arthritis (RA). In order to further dissect this issue, we numerically and graphically modeled DAS28, SDAI and CDAI by 3-dimensional plotting. We wished to graphically visualize the relative contribution of various elements in the three activity indices to each other.
Methods.We calculated DAS28 (3-variable), SDAI and CDAI by the standard equations. We plotted 3-dimensional (3D) "carpets" showing all combinations of the corresponding variables yielding to DAS28=5.1, DAS28=3.2, DAS28=2.6, SDAI=26, SDAI=11 and SDAI=3.3. We also plotted the 3D carpet for CDAI.Results: In patients with high or moderate disease activity, erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP) were not major confounding factors when calculating DAS28 and SDAI, respectively. In contrast, ESR and CRP highly overshadowed changes in joint counts and global assessments in patients with low disease activity (LDA) or in those in remission. No reliable assessment of LDA can be performed in cases where ESR>54 mm/h or CRP>20 mg/dl. Similarly, remission cannot be determined if ESR>19 mm/h or CRP>5 mg/dl.As CDAI does not include acute phase reactants, CDAI may be a useful tool even in states of remission or LDA.
Conclusions:Our results suggest that acute phase reactnats are indeed major confounding factors and should be omitted when assessing RA disease activity, at least in special cases.