RESUMEN
This article reviews the characteristics and weaknesses of the rheumatoid factor (RF) assay compared with anti-citrullinated peptide antibody (ACPA) testing in the work-up of patients with synovitis. This should lead physicians to change their ordering habits and replace RF by ACPA. For RA diagnosis, good clinical judgement based on clinical history, physical examination and routine laboratory work exceeds the value of RF and ACPA assays. In settings of both low and high pretest probability, the added value of each of these assays is low. In cases with intermediate probability, ACPA assays are superior to immunoglobulin (Ig)M-RF because of their higher specificity, and they should be the first choice in a RA diagnostic work-up. Dual testing brings few additional advantages and increases costs significantly. ACPA and IgM-RF are both imperfect tests; around 30% of patients with manifest RA will test negative in both assays and therefore caution needs to be exercised when interpreting negative results. Since 2009, the anti-cyclic citrullinated peptide (anti-CCP) antibody assay has been the only assay available at our institution for RA work-up, with IgM-RF available on a case-by-case basis for non-RA diseases. This has led to a 70% reduction in RF assays performed annually.