What Does the Anti-CCP Test Measure? Cyclic Citrullinated Peptide Antibodies

The anti-cyclic citrullinated peptide test, usually shortened to anti-CCP or written as cyclic citrullinated peptide IgG or cyclic citrullinated peptide Ab, is a laboratory immunoassay performed on a serum sample. It measures antibodies, most commonly of the immunoglobulin G class, that bind to short synthetic peptides engineered to contain the amino acid citrulline. Reviews of rheumatoid arthritis serology group these antibodies under the broader label anti-citrullinated protein antibodies, abbreviated ACPA, and describe the CCP assay as the most widely used laboratory method of capturing that antibody family (PMID 34054878). This page summarises what the published literature says the test measures and what it does not. This page is for educational purposes only and is not medical advice; consult a licensed physician about any laboratory result, symptom or health decision.

What a Citrullinated Peptide Is

Proteins are chains of amino acids joined by peptide bonds. After a protein is built, enzymes can chemically alter individual amino acid residues, a process called post-translational modification. Citrullination is one such modification: peptidylarginine deiminase enzymes convert an arginine residue inside the chain into citrulline. The change removes a positive charge from the side chain and alters the local shape and chemistry of the peptide. Reviews of rheumatoid arthritis autoantibodies describe this altered residue as the structural feature that the relevant antibodies recognise, meaning the immune reaction is directed at a modified self peptide rather than at the unmodified original (PMID 34054878). A 2010 review of the test explained that commercial assays present these citrulline-containing sequences in a cyclic, ring-closed conformation, because that geometry was found to improve antibody binding compared with linear peptides (PMID 20727426).

The word "cyclic" in the test name therefore refers to the shape of the synthetic peptide used as bait on the assay plate, not to anything cyclical about the patient or the disease. Readers who want background on the underlying chemistry can review the peptide bond glossary entry.

What the Assay Actually Detects

In a typical enzyme-linked immunosorbent assay format, the citrullinated peptide is fixed to a solid surface, diluted serum is added, and any antibodies in the sample that recognise the peptide remain bound after washing. A labelled secondary reagent directed at human IgG then generates a signal proportional to the amount of bound antibody. A 2010 review noted that successive assay generations, often labelled CCP2 and CCP3, used different peptide panels, and that this is one reason results are not freely interchangeable between manufacturers (PMID 20727426). A 2019 overview of rheumatoid arthritis serologic diagnosis likewise described the available assay platforms and emphasised that laboratories establish their own cut-off values and units (PMID 31668267).

For that reason no universal reference range is stated here. The numeric threshold that separates negative from positive, and the units in which the result is expressed, depend on the kit and the performing laboratory, and the report issued by that laboratory carries the applicable range. Interpretation belongs to the ordering clinician.

Published Role in Rheumatoid Arthritis Assessment

Reviews of rheumatoid arthritis serology describe anti-CCP as one of the two serologic markers used in contemporary classification of the disease, the other being rheumatoid factor, with antibody status contributing to the overall classification score alongside joint involvement, acute phase reactants and symptom duration (PMID 31668267). A 2022 clinical relevance review similarly positioned ACPA testing within the diagnostic workup and discussed its use in distinguishing inflammatory arthritis subtypes (PMID 34926155).

Beyond classification, several papers examined the marker in early disease. A 2008 paper described anti-CCP antibody as a marker relevant to the early detection of rheumatoid arthritis (PMID 19076355), and a 2007 review examined anti-CCP testing as both a diagnostic and a prognostic tool, reporting an association between antibody positivity and a more erosive disease course (PMID 17434910). A 2020 study of patients with rheumatoid arthritis evaluated anti-CCP antibody as a predictor of disease complications (PMID 32801432). None of these papers describe the test as a treatment, and none of the findings summarised here constitute guidance for any individual.

Sensitivity and Specificity as Reported in the Literature

Two properties define how a laboratory test behaves. Sensitivity is the proportion of people who truly have the condition that the test flags as positive. Specificity is the proportion of people without the condition that the test correctly leaves negative. Reviews comparing rheumatoid factor and anti-CCP consistently reported that the two markers show broadly comparable sensitivity for rheumatoid arthritis, while anti-CCP showed higher specificity, meaning fewer positive results among people who do not have the disease (PMID 18335856). A 2010 review reached the same general conclusion and discussed the clinical consequences of that specificity advantage (PMID 20727426).

A 2015 meta-analysis pooled diagnostic accuracy data for anti-CCP and for anti-mutated citrullinated vimentin, and researchers reported that both markers performed well for rheumatoid arthritis while differing in the balance between sensitivity and specificity (PMID 26111961). Exact pooled percentages vary by the assay generation, the control group and the cut-off used in each included study, which is why no single number is reproduced here as if it applied to every laboratory.

How Anti-CCP Is Read Alongside Rheumatoid Factor

Rheumatoid factor is a different analyte. It is an antibody directed at the constant region of other antibodies rather than at a citrullinated peptide, and it appears in a range of infections and other inflammatory conditions. A 2008 review set the two markers side by side and reported that rheumatoid factor positivity occurs in a wider set of non-rheumatoid conditions, which is the main reason anti-CCP carries greater specificity for rheumatoid arthritis (PMID 18335856). A 2019 serologic diagnosis review described the common practice of ordering both markers, because the combination yields different information than either alone (PMID 31668267).

• Both positive: reviews describe this combination as the one most strongly associated with rheumatoid arthritis in symptomatic patients (PMID 31668267).
• Anti-CCP positive, rheumatoid factor negative: the 2010 review discussed anti-CCP detection in patients negative for rheumatoid factor (PMID 20727426).
• Rheumatoid factor positive, anti-CCP negative: reviews note that rheumatoid factor alone is less specific because it occurs in other conditions (PMID 18335856).
• Both negative: reviews describe seronegative rheumatoid arthritis, in which the clinical diagnosis rests on other findings (PMID 34926155).

What a Positive or Negative Result Can and Cannot Show

A positive anti-CCP result reports that IgG binding to the citrullinated peptide exceeded the laboratory cut-off. It is a laboratory observation, not a diagnosis in itself. The predictive value of any positive result depends on how common the condition is in the group being tested, a point examined directly in a 2020 analysis which questioned whether measuring ACPA in the general population or in unselected primary care patients was useful and reported that testing outside an appropriate clinical context produced results that were difficult to act on (PMID 32079664).

A negative result reports that measurable binding fell below the cut-off on that assay at that time. Because sensitivity is incomplete, reviews describe patients with clinically evident rheumatoid arthritis who remain antibody negative (PMID 34926155). A negative value therefore does not exclude inflammatory joint disease, and a positive value does not establish it. The 2021 review of rheumatoid arthritis autoantibodies set out the laboratory and clinical caveats that accompany result interpretation (PMID 34054878).

Other Populations Studied

Anti-citrullinated peptide antibodies have also been evaluated outside adult rheumatoid arthritis. A 2016 study assessed the diagnostic performance of ACPA in juvenile idiopathic arthritis and reported that test characteristics in that paediatric setting differed from those described in adult disease (PMID 27323035). Research into additional citrullinated targets also continues: a 2025 large-scale multicenter study reported an anticitrullinated SR-A peptide antibody as a candidate biomarker in rheumatoid arthritis (PMID 39752500). That antigen is a research target described in the study and is distinct from the standard commercial CCP assay.

Interpretation Limits: What Studies Report

Published discussions of the assay repeatedly return to several limitations. Cut-off values and units are assay specific, so a numeric value from one laboratory should not be compared directly with a value from a different platform, a point raised in the 2010 review of the test (PMID 20727426). Pre-test probability matters, and researchers reported that applying the test in unselected populations complicates interpretation (PMID 32079664). Antibody status has also been studied in relation to prognosis rather than diagnosis alone, with one review reporting an association between positivity and joint damage over time (PMID 17434910) and a 2020 study examining positivity in relation to disease complications (PMID 32801432). None of this literature supports self-interpretation of a result, and none of it describes the test as a measure of general health.

Related Reading

• Peptide bond: the chemical link that defines a peptide
• Peptide hormones and how the body signals with short chains
• What the literature says about peptide safety

References

• Autoantibodies in Rheumatoid Arthritis: Laboratory and Clinical Perspectives (Frontiers in Immunology, 2021)
• All you wanted to know about anti-CCP but were afraid to ask (Autoimmunity Reviews, 2010)
• Rheumatoid factor and anti-CCP autoantibodies in rheumatoid arthritis: a review (Clinical Laboratory Science, 2008)
• Measuring ACPA in the general population or primary care: is it useful? (RMD Open, 2020)
• Diagnostic accuracy of anti-MCV and anti-CCP antibodies in rheumatoid arthritis: A meta-analysis (Zeitschrift fur Rheumatologie, 2015)
• Anti-cyclic citrullinated peptide antibody (ACPA) and Rheumatoid arthritis: Clinical relevance (Journal of Clinical Orthopaedics and Trauma, 2022)
• Serologic Diagnosis of Rheumatoid Arthritis (Clinics in Laboratory Medicine, 2019)
• Anti-Cyclic Citrullinated Peptide Antibody as a Predictor of Rheumatoid Arthritis Complications (Medical Archives, 2020)
• Anti-CCP Antibody, a Marker for the Early Detection of Rheumatoid Arthritis (Annals of the New York Academy of Sciences, 2008)
• Anti-CCP antibody testing as a diagnostic and prognostic tool in rheumatoid arthritis (QJM, 2007)
• Diagnostic performance of anti-citrullinated protein/peptide antibodies in juvenile idiopathic arthritis (Genetics and Molecular Research, 2016)
• Large-scale multicenter study reveals anticitrullinated SR-A peptide antibody as a biomarker and exacerbator for rheumatoid arthritis (Science Advances, 2025)