What it is
A cyclic citrullinated peptide is a piece of laboratory equipment, not a medicine. It is a short synthetic peptide with two defining features:
- It contains citrulline, an amino acid that is not built into proteins directly. It appears when the side chain of arginine is chemically converted — a change called deimination (or citrullination). The original assay paper describes its substrate as containing "deiminated arginine (citrulline)" (PMID 10643712).
- It is cyclic — closed into a ring. The 2000 paper that introduced the test describes designing a cyclic citrulline-containing variant specifically as the antigen for it.
Nobody takes it. Its entire use is to sit on a test plate and catch antibodies.
What the test does with it
In the anti-CCP test, a blood sample is exposed to the peptide. Some people's immune systems make antibodies against citrullinated proteins; those antibodies stick to the peptide and are measured. The result reports the antibodies, not the peptide. The peptide is the bait; the antibody is the catch.
The first version was described in January 2000 by Schellekens and colleagues in Arthritis & Rheumatism (PMID 10643712), who designed the cyclic citrulline-containing peptide for exactly this purpose. Later versions of the test use peptides chosen by the kit manufacturers, which is why the sequence is not a single published molecule.
What the studies report
These are published accuracy figures for the test as a whole, reported as the sources state them. They are not a way to read an individual result.
| Source | Group | Specificity | Sensitivity |
|---|---|---|---|
| Schellekens et al. 2000 (PMID 10643712) | Established rheumatoid arthritis vs other sera | 98% | 68% |
| Same paper | Early-arthritis clinic | 96% | 48% |
| Nishimura et al. 2007 meta-analysis, 37 studies (PMID 17548411) | Pooled | 95% (94–97%) | 67% (62–72%) |
| Same meta-analysis — IgM rheumatoid factor, for comparison | Pooled | 85% (82–88%) | 69% (65–73%) |
In plain terms: specificity is how often people without the disease test negative, and sensitivity is how often people with it test positive. A test with 95% specificity and 67% sensitivity rarely flags people without rheumatoid arthritis but misses roughly a third of people who have it. The meta-analysis also found that in three of four studies, anti-CCP positivity was more strongly linked than rheumatoid factor to later joint damage visible on X-ray. Its authors noted that many included studies had methodological limitations.
A result is interpreted by the clinician who ordered it, alongside symptoms, examination and other tests. This entry reports what the studies found across groups; it does not interpret anyone's result.
Why it is in this library
Because the word "peptide" on a lab report sends people to the same searches as injectable peptides. The two have nothing in common beyond the word: a cyclic citrullinated peptide has no dose, no route, no supplier and no effect on the person whose blood is tested. For what peptides are as molecules, see the peptide chain; for arginine and citrulline's place among the amino acids, see the amino acid chart.
Where it stands
PubMed indexes 5,295 records for anti-CCP or cyclic citrullinated peptide (2026-09-30), almost all about the antibody test and rheumatoid arthritis. There is no regulatory status to report for the peptide itself: it is a reagent inside diagnostic kits, which are regulated as tests.
Related reading
For peptides that the body itself makes and laboratories measure, see C-peptide and BNP and NT-proBNP. For the question most readers of this library arrive with — what the peptides sold online actually are — start at what are peptides.