Background Disease activity in rheumatoid arthritis (RA) has traditionally been measured using the 28-joint count disease activity score (DAS28) using ESR. Use of DAS28 with CRP in place of ESR is increasing. Evidence suggests these scores are not truly interchangeable and agreement varies dependant on disease activity. This study uses the BSRBR-RA to investigate agreement between these scores, proposing an optimised method to calculate the estimated DAS28-ESR using CRP (mDAS28-CRP) that offers higher interscore agreement than currently.

Objectives To investigate and optimise agreement between the DAS28-ESR and DAS28-CRP scores using the BSRBR-RA.

Methods Patients with concurrent measures of ESR and CRP were identified from the BSRBR-RA, enabling paired calculation of DAS28-ESR and DAS28-CRP using existing formulae. A non-linear regression model enabled prediction of ESR from CRP. Resulting values were used in the existing DAS28-ESR formula to calculate estimated disease activity (mDAS28-CRP). A Bayesian approach was used to acknowledge the uncertainty associated with the predictions and correctly propagate it through to mDAS28-CRP. Inference, including constructing credible intervals for predicted ESR and mDAS28-CRP, was performed using Markov-Chain-Monte-Carlo. Bland-Altman analysis and agreement matrices were used to compare agreement levels between mDAS28-CRP, DAS28-CRP and DAS28-ESR.

Results 5457 patients (mean age 56 yrs, 76% female) with 31,084 data entries were identified where paired DAS28-ESR/DAS28-CRP scores could be calculated. Mean DAS28-ESR was 0.3 points greater than DAS28-CRP (4.4 (SD 1.7), 4.1 (SD 1.6) respectively) compared with mean mDAS28-CRP of 4.6 (SD 1.6; Fig 1). mDAS28-CRP had superior agreement with DAS28-ESR across remission, low and moderate disease activity, with only a 6.8% reduction in agreement at high disease activity when compared with the DAS28-CRP (Table 1).

• Download figure
• Open in new tab
• Download powerpoint

Conclusions The mDAS28-CRP (online calculator pending) allows calculation of a disease activity score, using the same component parts of the DAS28-CRP, and has higher agreement levels with the original validated DAS28-ESR, than the current DAS28-CRP in this cohort. Disease activity scores are fundamental in clinical trials and practice, forming a central role in guiding availability of therapeutic agents. It is therefore essential that scores used are both valid and, if used interchangeably, have high agreement levels.

Disclosure of Interest None declared