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 using C-reactive protein (CRP) in place of ESR is increasing. This study investigates the level of agreement between the DAS28-ESR and DAS28-CRP scores across different disease activity thresholds and identifies how patient characteristics may influence agreement.
Objectives To identify the interscore agreement between the DAS28-ESR and DAS28-CRP scores and identify if gender or body mass index (BMI) influence the level of agreement.
Methods Patients with concurrent measures of ESR and CRP were identified from the BSRBR-RA, enabling paired calculation of DAS28-ESR and DAS28-CRP. Paired scores were stratified by patients' baseline BMI and gender. Agreement between the scores was compared using Bland-Altman statistics and agreement matrices.
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 (95% CI -0.8 - 1.4) greater than DAS28-CRP (4.4 (SD 1.7) and 4.1 (SD 1.6) respectively). Men had a lower mean difference between the two scores compared with women (DAS28-ESR > DAS28-CRP by 0.2 points (95% CI -1.0 – 1.3) vs. 0.4 points (95% CI -0.7 – 1.4) respectively). The results stratified by BMI were similar to the overall mean difference. Agreement between the two scores according to disease activity thresholds are shown in Table 1.
Conclusions Overall, the DAS28-ESR classifies fewer patients in remission (15.6% vs. 19.5%) giving a score, on average 0.3 points greater than the DAS28-CRP, with women having a greater difference between the two scores than men. When categorising scores by disease activity thresholds, the DAS28-ESR/DAS28-CRP have lowest agreement at LDA. 54.4% of DAS28-ESR scores were classified as MDA when the paired DAS28-CRP was LDA, which could influence results in clinical trial reporting. Conversely, 20% of patients were classified as being in MDA by DAS28-CRP when the paired DAS28-ESR demonstrated HDA. This is of importance given NICE biologics guidelines, and shows that up to 20% of patients may not satisfy the criteria for biologic therapy if DAS28-CRP were used instead of DAS28-ESR. These results highlight the impact of using the DAS28-ESR or DAS28-CRP interchangeably, and the importance of using a consistent version of the DAS28.
Disclosure of Interest None declared