Background According to the 2010 criteria, rheumatoid arthritis (RA) can be classified in the presence of ≥6 points on the criteria or ‘typical’ erosive disease. RA-specific erosiveness however has not been defined yet. This study reports the results of the data driven phase of a European League Against Rheumatism (EULAR) taskforce aiming to define RA-specific erosiveness.

Methods Baseline radiographs of hands and feet of 980 Dutch and 811 French early arthritis patients were studied on the number and site of erosive joints. Test characteristics were determined, with the outcome measures being initiation of methotrexate (MTX) therapy or any disease modifying antirheumatic drug (DMARD) therapy within the first year of disease and arthritis persistency over 5 years. Analyses were repeated in the patients with <6 points on the American College of Rheumatology/EULAR 2010 criteria.

Results In both cohorts comparable test characteristics were observed for the outcomes MTX therapy, any DMARD therapy and arthritis persistency. Test characteristics were not influenced by the site of erosiveness. The specificity observed was >50% for ≥1 erosive joint, >80% for ≥3 erosive joints and >90% for ≥5 erosive joints. When analysing the patients not fulfilling the 2010 criteria (n=308 and 149), specificity was >60% for ≥1 erosive joint, >90% for ≥3 erosive joints and >95% for ≥5 erosive joints. Few of these patients fulfilled the radiological criterion; 27–36 patients had ≥3 erosive joints and 13–14 patients had ≥5 erosive joints.

Conclusions RA-specific erosiveness can be defined with high specificity at several cut-offs for the number of erosive joints in two independent cohorts with multiple different outcomes. The final radiological criterion will be established in the next phase.