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

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.

Keywords: Conversion equations; DAS28CRP; DAS28ESR; Disease activity; Rheumatoid arthritis; SDAI.