How much does Disease Activity Score in 28 joints ESR and CRP calculations underestimate disease activity compared with the Simplified Disease Activity Index?

Objectives Disease Activity Score in 28 joints calculated with C-reactive protein (DAS28-CRP) is used instead of erythrocyte sedimentation rate (DAS28-ESR) to assess rheumatoid arthritis disease activity; however, values for remission and low disease activity (LDA) for DAS28-CRP have not been validated. American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) guidelines suggest remission should be calculated by Simplified Disease Activity Index (SDAI) rather than DAS28-ESR. We examined values of remission and LDA of DAS28-CRP that correspond to the respective cut-off points for DAS28-ESR and SDAI from five clinical trials. Methods DAS28-CRP cut-offs that best correspond to DAS28-ESR remission <2.6 and LDA 3.2 were obtained by cumulative distribution plots, receiver operating curves and maximum concordance and averaged for each approach, treatment group and study. Level of agreement between DAS28-CRP and DAS28-ESR remission and LDA cut-offs was compared against each other and versus SDAI remission 3.3 and LDA 11. Results Percentage of patients who achieved remission and LDA by DAS28-ESR cut-offs was greater for DAS28-CRP versus DAS28-ESR regardless of patient population or treatment group. Discordance between CRP and ESR cut-offs ranged from 4%-26% and 8%-23% for remission and LDA, respectively, and 19%-40% and 6%-11% for DAS28-CRP versus SDAI, respectively. Estimated (range) remission and LDA thresholds were 2.4 (2.2-2.6) and 2.9 (2.6-3.3), 1.9 (1.6-2.2) and 3.1 (3.1-3.3) and 2.2 (1.1-2.9) and 3.6 (3.4-4.0) for DAS28-CRP versus DAS28-ESR, DAS28-CRP versus SDAI and DAS28-ESR versus SDAI, respectively. Conclusions DAS28-CRP underestimates disease activity when using cut-off points validated for DAS28-ESR; therefore, DAS28-ESR cut-off values should not be applied to DAS28-CRP. Although DAS28-CRP and DAS28-ESR cut-offs for LDA 3.2 correspond to SDAI LDA, neither corresponds well to SDAI remission.