Rheumatoid factors (RFs) are autoantibodies directed against the Fc portion of IgG. Tumour markers (eg, CA-125, CA-19-9) are tumour-expressed proteins that are usually measured in the serum by means of ELISA sandwich assays, in which murine monoclonal antibodies are used to capture the circulating antigen under investigation.^1–3

Given the extensive homology between the Fc regions of murine and human IgG antibodies, it may not be unexpected that anecdotal reports have shown that tumour markers can be falsely increased in the presence of human antimouse antibodies or RFs.^4–6 We systematically sought to determine whether patients with active RF-positive rheumatoid arthritis …