anticitrullinated

Anticitrullinated refers to antibodies directed against citrullinated proteins or peptides. In immunology and rheumatology the term is most often encountered as anticitrullinated protein antibodies (ACPAs) or anti-citrullinated peptide antibodies (ACPA). Citrullination is a post-translational modification in which the enzyme peptidyl arginine deiminase converts arginine residues in proteins into citrulline, generating neoepitopes that can be recognized by the immune system, particularly during inflammatory processes.

ACPAs are strongly associated with rheumatoid arthritis (RA). They are highly specific for RA, with specificity commonly cited around 95% for RA vs other rheumatic diseases, though sensitivity varies (roughly 60-80% depending on the assay). In many patients, ACPAs appear years before the onset of clinical arthritis and can assist early diagnosis when used in combination with clinical assessment and other tests such as rheumatoid factor.

Testing for ACPAs typically uses anti-cyclic citrullinated peptide (anti-CCP) assays or other ACPA platforms. Anti-CCP tests detect antibodies to synthetic citrullinated peptides and have become standard in RA workups due to their high specificity. Some patients are ACPA-negative but RA-positive; conversely, ACPA positivity may be observed in other conditions but is uncommon.

Clinically, ACPA status has prognostic value in RA; positivity is associated with an increased risk of radiographic joint damage and may influence decisions about treatment intensity. The term "anticitrullinated" thus describes antibodies targeting citrullinated proteins and is central to understanding a key diagnostic and prognostic marker in rheumatoid arthritis.