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Development and validation of equations for conversion from DAS28ESR and DAS28CRP to the SDAI in patients with rheumatoid arthritis

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A LETTER TO THE EDITOR to this article was published on 06 April 2024

Abstract

Introduction

Disease Activity Score-28 (DAS28) with erythrocyte sedimentation rate (DAS28ESR), DAS28 with C-reactive protein (DAS28CRP), and simplified disease activity index (SDAI) are widely used to assess disease activity as low, moderate, or high or in remission in patients with rheumatoid arthritis (RA). However, these indicators can generate inconsistent results, influencing treatment decisions and limiting comparisons across studies. We aimed to establish equations for conversion from DAS28ESR and DAS28CRP to SDAI.

Methods

We conducted a retrospective study, including 933 outpatients who were simultaneously assessed using DAS28ESR, DAS28CRP, and SDAI. The patients were divided into a training set (70%) and a validation set (30%). We developed equations to convert DAS28ESR and DAS28CRP values into SDAI values by bisquare-weighted robust regression to obtain SDAI–DAS28ESR and SDAI–DAS28CRP. In addition to using kappa values to assess consistency, differences in disease activity classification between SDAI–DAS28ESR and SDAI–DAS28CRP were examined by the Stuart–Maxwell test and the Bowker test.

Results

Two quadratic equations were developed as follows: SDAI–DAS28ESR = 1.168 × (DAS28ESR)^2 − 2.432 × (DAS28ESR) + 2.649 and SDAI–DAS28CRP = 1.2 × (DAS28CRP)^2 − 0.3522 × (DAS28CRP) − 0.6014. After applying the equations, the Stuart–Maxwell test and the Bowker test were no longer significant between SDAI–DAS28ESR and SDAI or between SDAI–DAS28CRP and SDAI. The kappa values increased from 0.57 to 0.73 between SDAI–DAS28ESR and SDAI and 0.76 to 0.86 between SDAI–DAS28CRP and SDAI.

Conclusion

SDAI–DAS28ESR and SDAI–DAS28CRP are interchangeable with the SDAI on the group level, which will facilitate comparisons among studies. In addition, the equations improved consistency between indicators.

Key Points

• There is disagreement in assessing disease activity in patients with rheumatoid arthritis between Disease Activity Score-28 (DAS28) with erythrocyte sedimentation rate (DAS28ESR), DAS28 with C-reactive protein (DAS28CRP), and simplified disease activity index (SDAI).

• We developed and validated two quadratic equations to convert DAS28ESR and DAS28CRP into SDAI. We found there was no longer significant difference in disease activity between indicators after applying the equations.

• This work may allow comparisons across studies which use different indicators.

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Acknowledgements

We would like to thank all the patients and healthcare professionals who participated in this study.

Funding

This work was supported by a research grant from the National Science and Technology Major Project for Major New Drugs Innovation and Development (no. 2018ZX09734003) and National Natural Science Foundation of China (no. 82171780).

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Author notes

  1. Li-ming Pu and Yin Liu contributed equally to the work

Authors and Affiliations

  1. Faculty of Medical Statistic，School of Public Health, Sun Yat-Sen University, Guangdong, Guangzhou, China

    Li-ming Pu, De-xin Zhou, Xi-jia Tang, Chao-nan Fan, Wen Chen & Li Ling

  2. The Affiliated Cancer Hospital of Zhengzhou University & Henan Cancer Hospital, Henan, Zhengzhou, China

    Yin Liu

  3. Department of Rheumatology, Sun Yat-Sen Memorial Hospital, Sun Yat-Sen University, Guangzhou, Guangdong, China

    Jian-da Ma, Jian-zi Lin & Lie Dai

  4. Clinical Research Center, Sun Yat-Sen Memorial Hospital, Sun Yat-Sen University, Guangzhou, Guangdong, China

    Li Ling

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Pu, Lm., Liu, Y., Zhou, Dx. et al. Development and validation of equations for conversion from DAS28ESR and DAS28CRP to the SDAI in patients with rheumatoid arthritis. Clin Rheumatol 41, 3697–3706 (2022). https://doi.org/10.1007/s10067-022-06259-z

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