PRTN3 and rheumatoid arthritis: Anti‐CarP tend to be found mainly in ACPA‐positive RA patients, but is also present in 8%‐14% of ACPA‐negative patients.69 Similar to ACPA, anti‐CarP can also be present years before disease onset.70 Furthermore, the anti‐CarP response shows isotype switching and is, like the ACPA response, of overall low avidity as compared to recall antigens.71