Anti-Cyclic Citrullinated Peptide Antibody

Anti-citrullinated protein antibodies (ACPA), measured as anti-cyclic citrullinated peptide (anti-CCP) antibodies, are highly specific autoantibodies for rheumatoid arthritis. They target proteins in which arginine has been converted to citrulline by peptidylarginine deiminase enzymes, a modification that occurs in inflamed synovium and in the lung of smokers.

Second- and third generation ELISA and chemiluminescence assays measure IgG anti-CCP2 (and CCP3). Anti-CCP is positive in 60 to 70% of rheumatoid arthritis and has specificity of 95 to 98%, so it is the best single serological test for diagnosis and for predicting severe, erosive disease.

Anti-CCP antibodies appear years before clinical onset (up to 10 years), and positive patients with arthralgia and no synovitis have a 30 to 50% chance of developing rheumatoid arthritis within 3 to 5 years. Together with RF, they form the serological component of the classification criteria. Smoking, silica exposure and periodontal disease (Porphyromonas gingivalis) are linked to ACPA production in people with HLA-DRB1 shared epitope alleles.

Indications. Suspected rheumatoid arthritis in a patient with inflammatory arthritis, particularly early (under 6 months) or when RF is negative. Undifferentiated arthritis to predict progression. Arthralgia with high suspicion. Assessment of prognosis. Not for monitoring disease activity.

Specimen. Serum 1 mL, no fasting. Stable for several days at 2 to 8 °C, or frozen. Avoid repeated freeze thaw cycles.

Reference values (assay dependent).

• Negative: under 20 U/mL (CCP2 ELISA; some assays under 5 or under 7 U/mL).
• Low positive: 20 to 60 U/mL (up to 3 times the upper limit).
• High positive: over 60 U/mL (over 3 times the upper limit).
• Cut offs vary by manufacturer, and the laboratory's range applies.

Interpretation.

• Positive anti-CCP in a patient with symmetrical small joint arthritis: rheumatoid arthritis is very likely (positive predictive value over 90%), with high titres and the RF positive adding certainty.
• Positive anti-CCP with negative RF: still specific for rheumatoid arthritis, and 10 to 15% of cases are RF-negative and anti-CCP-positive.
• Negative anti-CCP: does not exclude (30 to 40% of rheumatoid arthritis is seronegative), and other diagnoses are considered.
• Anti-CCP high titres predict radiographic erosions (relative risk 2 to 5), faster progression, interstitial lung disease and cardiovascular risk, and support earlier use of disease modifying drugs.
• Early arthritis: anti-CCP positive undifferentiated arthritis has 70 to 90% chance of becoming rheumatoid arthritis.
• Response to treatment: anti-CCP levels often fall slowly but change little, and do not track disease activity.

Other conditions with positive anti-CCP (usually low titre).

• Psoriatic arthritis (5 to 15%), Sjögren syndrome (5 to 10%), systemic lupus erythematosus (5 to 10%, associated with erosive arthropathy such as Jaccoud), tuberculosis, chronic hepatitis C, interstitial lung disease, and healthy relatives of patients (about 1 to 3% of the general population).
• Reported in primary biliary cholangitis, chronic obstructive lung disease and in some elderly people.

Critical values and findings needing immediate action.

• Positive anti-CCP with synovitis lasting over 6 weeks: early referral to rheumatology, disease modifying therapy within 3 months of onset.
• Positive anti-CCP with cough, dyspnoea or crackles: interstitial lung disease, high resolution CT and lung function.
• Positive anti-CCP with systemic features (fever, weight loss, nodules, neuropathy): extra-articular disease or vasculitis.
• Positive anti-CCP in a patient planning anti-TNF therapy: screen for tuberculosis and hepatitis B.

Interferences and pitfalls.

• Assays differ in sensitivity and specificity, and results are not interchangeable. Third generation (CCP3) assays raise sensitivity slightly with lower specificity.
• Low titre positives (up to 3 times the cut off) in a patient without clinical arthritis are of limited value, and clinical follow up rather than treatment is needed.
• Heterophile antibodies and rheumatoid factor can interfere with immunoassays, giving false positives.
• The test does not replace clinical assessment, imaging (ultrasound showing synovitis, MRI bone oedema, radiographic erosions) and the classification criteria.
• Anti-CCP may be negative in early disease and appear later, and repeat testing after 6 to 12 months can convert from negative to positive.
• Repeating anti-CCP to monitor treatment response has little value.
• Positive results in people at risk (first degree relatives, smokers) do not require treatment but call for monitoring.
• Smoking cessation and periodontal care reduce risk and may improve response to therapy.

Special groups.

• Early arthritis clinics: anti-CCP, RF, ESR, C-reactive protein and imaging are combined to inform the decision to start treatment.
• Women: more frequent disease, with pregnancy improving symptoms in some.
• Older onset rheumatoid arthritis: anti-CCP is less frequent, and polymyalgia rheumatica is a competing diagnosis.
• Children: anti-CCP is positive in the polyarticular seropositive type of juvenile arthritis.
• Regions: lower prevalence of anti-CCP in some African populations, and infections may produce false positives.

Follow up tests. Rheumatoid factor, ESR and C-reactive protein, full blood count, liver and renal function, hand and foot radiographs, musculoskeletal ultrasound or MRI, ANA, hepatitis B and C and HIV screening, tuberculosis screening before biologic treatment, high resolution CT chest if respiratory features, HLA-DRB1 typing in research, and cardiovascular risk assessment.