Role of Anti–Carbamylated Protein Antibodies Compared to Anti–Citrullinated Protein Antibodies in Indigenous North Americans With Rheumatoid Arthritis, Their First‐Degree Relatives, and Healthy Controls

Objective Rheumatoid arthritis (RA) is characterized by the presence of autoantibodies, including seropositivity for rheumatoid factor (RF) and anti–citrullinated protein antibodies (ACPAs). In addition, antibodies to carbamylated proteins (anti‐CarP) are present in patients with RA and are associated with joint damage. This study was undertaken to assess the presence of anti‐CarP antibodies in indigenous North Americans (First Nations [FN] populations) with RA compared to their at‐risk first‐degree relatives (FDRs) and healthy controls. Methods Anti‐CarP IgG and ACPAs (specifically, anti–cyclic citrullinated peptide [anti‐CCP] antibodies) were measured by enzyme‐linked immunosorbent assay in the sera of FN patients with RA (n = 95), their unaffected FDRs (n = 109), and healthy FN controls (n = 85). Antibodies to additional citrullinated peptides were measured using a multiplex ACPA array, and the number of peptides recognized was reported as an ACPA score. Groups were compared using the chi‐square test and Mann‐Whitney U test. Associations between RA and seropositivity for RF, ACPAs, and anti‐CarP antibodies were determined by logistic regression. Results Anti‐CarP antibodies were more frequent in FN patients with RA (44.3%) compared to FDRs (18.3%) and FN controls (4.7%) (both P