PRTN3 and rheumatoid arthritis: Multiple ACPA‐seropositive patients with RA, ACPA‐seronegative patients with RA, and controls showed an OD >1 to the citrullinated peptide in their stool but also to unmodified arginine‐containing peptide, which was thus considered as nonspecific binding/AMPA negative (Figure 5A and D).