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ACPA – Antibodies in Rheumatoid Arthritis

ACPA (Anti-Citrullinated Protein Antibodies) are blood markers used for the early diagnosis of rheumatoid arthritis. Their detection is considered highly specific for this autoimmune disease.

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Things worth knowing about "ACPA"

ACPA (Anti-Citrullinated Protein Antibodies) are blood markers used for the early diagnosis of rheumatoid arthritis. Their detection is considered highly specific for this autoimmune disease.

What are ACPA?

ACPA stands for Anti-Citrullinated Protein Antibodies. These are autoantibodies produced by the immune system that mistakenly target the body's own proteins containing the amino acid derivative citrulline. Citrulline is formed through a chemical modification process called citrullination, in which the amino acid arginine within proteins is converted. In clinical practice, ACPA are most commonly detected using the Anti-CCP test (Anti-Cyclic Citrullinated Peptide).

Clinical Significance

ACPA are among the most important biomarkers for diagnosing rheumatoid arthritis (RA), a chronic inflammatory joint disease. Their presence in the blood is highly specific for RA and can be detected years before clinical symptoms appear, making ACPA a valuable tool for early detection.

Specificity for rheumatoid arthritis: over 95%
Sensitivity: approximately 60–70%
ACPA-positive patients often experience a more severe disease course

How is the ACPA Test Performed?

ACPA are measured from a simple blood sample. In the laboratory, Anti-CCP antibodies are quantified using an immunological assay method called ELISA (Enzyme-Linked Immunosorbent Assay). Results are reported in units per milliliter (U/ml), and values above the laboratory-specific threshold are considered positive.

Diagnosis of Rheumatoid Arthritis

ACPA testing is part of the ACR/EULAR classification criteria for rheumatoid arthritis (2010). A positive ACPA result, combined with clinical signs such as joint swelling, morning stiffness, and elevated inflammatory markers (CRP, ESR), strongly supports the diagnosis.

Differentiation: ACPA vs. Rheumatoid Factor

In addition to ACPA, the rheumatoid factor (RF) is often measured when rheumatoid arthritis is suspected. ACPA are considered more specific than the rheumatoid factor, as RF can also be elevated in other conditions. The combination of both markers improves diagnostic accuracy.

Prognostic Value

A positive ACPA result carries not only diagnostic but also prognostic significance. Compared to ACPA-negative patients, those who test positive face a higher risk of:

Erosive joint destruction
More severe disease progression
Reduced response to certain therapies

Therefore, ACPA status influences both the choice of treatment and the assessment of long-term outcomes.

Treatment Relevance

While ACPA themselves do not require direct treatment, their detection helps physicians initiate early and targeted therapy for rheumatoid arthritis. Treatment options include disease-modifying antirheumatic drugs (DMARDs) such as methotrexate, as well as biologics that specifically modulate immune system activity.

References

Aletaha D et al. - 2010 Rheumatoid Arthritis Classification Criteria: An American College of Rheumatology/European League Against Rheumatism Collaborative Initiative. Arthritis & Rheumatism, 2010.
Schellekens GA et al. - The diagnostic properties of rheumatoid arthritis antibodies recognizing a cyclic citrullinated peptide. Arthritis & Rheumatism, 2000.
Smolen JS et al. - EULAR recommendations for the management of rheumatoid arthritis with synthetic and biological disease-modifying antirheumatic drugs. Annals of the Rheumatic Diseases, 2020.

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