Objectives Rheumatoid factor (RF) and anti-cyclic citrullinated protein/peptide antibodies (ACPA) are integrated in the 2010 American College of Rheumatology/European League Against Rheumatism (ACR/EULAR) classification criteria for rheumatoid arthritis (RA). The objectives of this study were to evaluate the technical and diagnostic performance of different RF and ACPA assays and to evaluate whether differences in performance impact RA classification.

Methods Samples from 594 consecutive patients who for the first time consulted a rheumatologist (44 of whom were diagnosed with RA) and 26 extra newly diagnosed patients with RA were analysed with six different RF assays (Menarini, Thermo Fisher, Inova, Roche, Abbott, Euroimmun) and seven different ACPA assays (Menarini, Thermo Fisher, Inova, Roche, Abbott, Euro Diagnostica, Euroimmun).

Results We found differences in analytical performance between assays. There was poor numerical agreement between the different RF and ACPA assays. For all assays, the likelihood ratio for RA increased with increasing antibody levels. The areas under the curve of receiver operating characteristic analysis of the RF (range 0.676–0.709) and ACPA assays (range 0.672–0.769) only differed between some ACPA assays. Nevertheless, using the cut-off proposed by the manufacturer, there was a large variation in sensitivity and specificity between assays (mainly for RF). Consequently, depending on the assay used, a subgroup of patients (13% for RF, 1% for ACPA and 9% for RF/ACPA) might or might not be classified as RA according to the 2010 ACR/EULAR criteria.

Conclusion Due to poor harmonisation of RF and ACPA assays and of test result interpretation, RA classification according to 2010 ACR/EULAR criteria may vary when different assays are used.