Citrulline Antibody Blood Test

Citrulline antibody is present in the blood of most patients with rheumatoid arthritis. It is used in the diagnosis of rheumatoid arthritis when evaluating patients with unexplained joint inflammation.

Citrulline antibody is an immune protein (antibody) that binds to a nonstandard amino acid (citrulline) that is formed by removing amino groups from the natural amino acid, arginine. A chemical process known as citrullination causes this change to occur. Citrulline antibodies are measured with a blood (serum) test that is analyzed in laboratories. The test is also known as an anti-citrulline antibody, cyclic citrullinated peptide antibody (CCPA), anti-cyclic citrullinated peptide antibody, and anti-CCP.

Research suggests that in the joints of patients with rheumatoid arthritis, proteins may be changed to citrulline as part of the process that leads to inflammation of the rheumatoid joint.

Citrulline antibody is present in the blood of most patients with rheumatoid arthritis. It is used in the diagnosis of rheumatoid arthritis when evaluating patients with unexplained joint inflammation. A test for citrulline antibodies is most helpful in looking for the cause of previously undiagnosed inflammatory arthritis when the substance detected by the traditional blood test for rheumatoid arthritis, rheumatoid factor, is not present. Citrulline antibodies have been felt to represent the earlier stages of rheumatoid arthritis in this setting and may be positive when the rheumatoid factor (RF) test is negative.

The citrulline antibody test then can be used by doctors to encourage them to proceed with the optimal treatments directed toward rheumatoid arthritis. Research has found that the presence of citrulline antibodies has also been associated with a greater tendency toward more destructive forms of rheumatoid arthritis.

The test is performed routinely in medical laboratories. Any physician can order the test to be done on a blood sample. Because the test is routinely performed for rheumatoid arthritis, pediatricians, internists, and rheumatologists are the physicians likely to order the test. The test is performed on a sample of venous blood collected by a routine blood draw.

Normal results for the citrulline antibody test (negative result) are <20 EU/mL.

Normal values (negative result) are <20 EU/mL. Positive results are classified as follows:

• 20-39 EU/mL -- weakly positive
• 40-59 EU/mL -- moderately positive
• >60 EU/mL -- strongly positive

When the citrulline antibody is found in a patient's blood, there is a 90%-95% likelihood that the patient has rheumatoid arthritis. Citrulline antibody can also be positive in certain patients with other connective tissue disorders, such as systemic lupus erythematosus. It is also present in some healthy people.