Origins of Discordant Responses among 3 Rheumatoid Arthritis Improvement Criteria.

Objective: We examined agreement between the American College of Rheumatology (ACR), European League Against Rheumatism (EULAR), and Simplified Disease Activity Index (SDAI) response criteria in rheumatoid arthritis (RA) and tested whether discordant responses were associated with patients' baseline characteristics or changes in RA activity encapsulated by the different criteria.

Methods: In a prospective longitudinal study, we examined responses of 243 patients with active RA to escalation of antirheumatic treatment. We computed agreement between pairs of response criteria using κ coefficients and identified patient characteristics associated with unique responses to individual criteria.

Results: We found that 110 patients (45.3%) had an ACR 20% improvement (ACR20) response, 135 (55.5%) had a EULAR moderate/good response, and 83 (34.1%) had an SDAI50 response. Agreement was moderate to good (ACR20/EULAR κ 0.57; ACR20/SDAI50 κ 0.64; EULAR/SDAI50 κ 0.59). All who had SDAI50 response also had a EULAR response. Patient characteristics at baseline generally did not distinguish those who responded to both, 1, or neither criterion. Discordance was most often because of improvements in the erythrocyte sedimentation rate or C-reactive protein level among EULAR and SDAI50 responders, which were not as common among ACR20 responders. Based on receiver-operating characteristic curves, SDAI35 response had a better balance of sensitivity and specificity relative to ACR20 and EULAR moderate/good responses than SDAI50.

Conclusion: Discordant responses to RA improvement criteria are most often because of differences in responses of acute-phase reactants. SDAI35 response had higher sensitivity for improvement, as reflected by other response criteria, than SDAI50 response.