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 28-joint disease activity scale (DAS28), simplified disease 
activity index (SDAI), and clinical disease activity index (CDAI) by 
three-dimensional (3D) plotting. We wished to graphically visualize the relative 
contribution of various elements in the three activity indices to each other. We 
calculated DAS28 (3 variables), SDAI, and CDAI by the standard equations. We 
plotted 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. In patients with high or 
moderate disease activity, erythrocyte sedimentation rate (ESR) or C-reactive 
protein (CRP) was not a major confounding factor 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 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. Our results 
suggest that acute phase reactants are indeed major confounding factors and 
should be omitted when assessing RA disease activity at least in special cases.