Citrulline antibody: An antibody (an immune protein) directed against a circular peptide (a ring of amino acids) containing an unusual ("non-standard") amino acid called citrulline that is not normally present in peptides or proteins. (Citrulline is formed by the body as an intermediary in the conversion of the amino acid orthithine to arginine). The citrulline antibody provides the basis for a test of importance in rheumatoid arthritis.

The citrulline antibody appears early in the course of rheumatoid arthritis and is present in the blood of most patients with the disease. When the citrulline antibody is detected in a patient's blood, there is a 90-95% likelihood that the patient has rheumatoid arthritis. The test for the citrulline antibody is therefore useful in the diagnosis of patients with unexplained joint inflammation, especially when the traditional blood test for rheumatoid factor is negative. The citrulline antibody also has prognostic (predictive) value since it is associated with a greater tendency toward more destructive forms of rheumatoid arthritis.

Citrullinated proteins have been discovered in the joints of patients with rheumatoid arthritis but not in other forms of joint disease. The citrullinated proteins in the joints correspond to the presence of the citrulline antibodies in the blood and suggest a possible role for these antibodies in the development of rheumatoid arthritis.

In sum, citrulline antibodies are valuable markers for the diagnosis and prognosis of rheumatoid arthritis for a number of reasons. Citrulline antibodies may be detected in healthy people long before they have clinical symptoms of rheumatoid arthritis. In both early and established rheumatoid arthritis, the test for citrulline antibodies is more specific than that for rheumatoid factor. Citrulline antibodies predict the eventual development of undifferentiated arthritis into rheumatoid arthritis. And citrulline antibodies are a marker of erosive joint disease in rheumatoid arthritis.

The citrulline antibody is known more formally as cyclic citrullinated peptide antibody. It may also be referred to as anti-citrulline antibody; anti-cyclic citrullinated peptide antibody; anti CCP; anti-CCP; cyclic citrullinated peptide antibody IgG; CCP IGG; and (nonspecifically) as a marker for rheumatoid arthritis.