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. 2020 Jul;7(3):105-111.
doi: 10.5152/eurjrheum.2020.19199. Epub 2020 Jul 1.

Conversion among the 28-joint count activity indices for rheumatoid arthritis

Khai Pang Leong  1 , Justina Wei Lynn Tan  1 , Xiao Gao  1 , Ee Tzun Koh  1 ; TTSH Rheumatoid Arthritis Study Group  1
Affiliations

Conversion among the 28-joint count activity indices for rheumatoid arthritis

Khai Pang Leong et al. Eur J Rheumatol. 2020 Jul.

Abstract

Objective: Disease activity indices for rheumatoid arthritis (RA) are important in clinical practice and research. Although they are closely correlated, they are not in good agreement. We derived formulae to convert values from one of the four 28-joint count indices (disease activity score using erythrocyte sedimentation rate [DAS28-ESR], disease activity score using C-reactive protein [DAS28-CRP], clinical disease activity index [CDAI], and simple disease activity index [SDAI]) to any of the others.

Methods: We obtained data from 175 patients from our RA registry with concurrent CRP and ESR and established the nature of relationships between the indices using these data. Subsequently, we developed empiric conversion formulae. Furthermore, we developed new cutoff values for classifying disease activity to minimize the disparity among indices, using an iterative method.

Results: The relationships between DAS28-ESR and DAS28-CRP and between SDAI and CDAI were approximately linear; the others were quadratic. Quadratic equations approximated the relationship between DAS, SDAI, and CDAI, whereas natural logarithms function approximated the relationship between DAS28-ESR and DAS28-CRP. Patients are frequently categorized into inconsistent disease activity states with any two indices, with the disparity ranging from 9.7% to 40.6%. The new cutoff values were developed to minimize the discrepant activity state categorization, reducing the disparity range to 6.3%-32.6%.

Conclusion: We derived empiric formulae that connect DAS28-ESR, DAS28-CRP, SDAI, and CDAI. Moreover, we developed new cutoff values to minimize the discrepant activity state categorization with different indices.

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Conflict of interest statement

Conflict of Interest: The authors have no conflict of interest to declare.

Figures

Figure 1. a, b
Figure 1. a, b
Bland-Altman plots showing the agreement between SDAI and CDAI (a) and between DAS28-ESR and DAS28-CRP (b). Each point represents a patient, displaying the difference in indices against the mean value of indices. The dotted lines represent the boundaries of the 95% limits of agreement. Points within the boundaries are considered to agree well.
Figure 2
Figure 2
The curve described by the equation DAS28-CRP ~ 1.1953e0.2487(DAS28-ESR) approximates the association between DAS28-CRP and DAS28-ESR.
Figure 3
Figure 3
The curve described by the equation CDAI=1.292(DAS28-ESR)2 - 3.132(DAS28-ESR) + 4.705 approximates the association between DAS28-ESR and CDAI.
See this image and copyright information in PMC

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