Anti-carbamylated Protein Antibodies Are Detectable in Various Connective Tissue Diseases

Objective.Anti-carbamylated protein (anti-CarP) antibodies are possible diagnostic biomarkers of anticitrullinated protein antibody (ACPA)-negative rheumatoid arthritis (RA). We aimed to elucidate the prevalence of anti-CarP antibodies in non-RA connective tissue diseases (CTD) because CTD are important in the differential diagnosis of ACPA-negative RA.Methods.The sera from 266 patients with RA and 616 patients with CTD and 80 healthy controls were examined using an in-house anti-CarP ELISA.Results.The prevalence and the level of anti-CarP antibodies in several CTD were comparable to those in ACPA-negative RA.Conclusion.Anti-CarP antibodies are not useful for differentiating ACPA-negative RA from CTD.