PRTN3 and rheumatoid arthritis: Having low antibody levels to all viruses was the most common combination (44% in controls; 46% in ACPA-negative RA; 53% in ACPA-positive RA), followed by low antibody levels to two viruses (i.e. high antibody levels to one virus - B19 in controls and ACPA-negative RA, and CMV in ACPA-positive RA).