In order to address the possibility that RF, which is more commonly detected in ACPA-positive RA than in ACPA-negative RA, was interfering with the detection of anti-viral antibodies and thereby caused lower anti-EBV and anti-B19 IgG levels in the ACPA-positive subset, we analysed the levels of IgM RF in relation to all three anti-viral antibody levels in all EIRA RA samples. This evidence concerns the gene PRTN3 and rheumatoid arthritis.