Anca Test: Purpose, Test Details & Results

Antineutrophil cytoplasmic antibody (ANCA) testing has become a cornerstone in diagnosing and managing certain types of vasculitis—diseases where the body’s immune system attacks its blood vessels. For patients with unexplained symptoms that could indicate small or medium vessel inflammation, the ANCA test helps clarify the diagnosis, guide treatment, and sometimes monitor disease activity. In this comprehensive article, we will explore the purpose of ANCA testing, how the test is performed, and how to interpret its results in the context of patient care.

Purpose of Anca Test

When patients present with symptoms such as unexplained inflammation, kidney problems, or respiratory issues, clinicians often consider a range of possible diagnoses. ANCA testing helps narrow down the possibilities, especially when certain forms of vasculitis are suspected. By detecting antibodies that target specific proteins inside neutrophils—a type of white blood cell—the test provides valuable clues about the underlying disease process.

Table 1: Clinical Purposes of ANCA Testing

Understanding the Clinical Need

ANCA testing is primarily utilized when clinicians suspect ANCA-associated vasculitides (AAV), which include:

• Granulomatosis with polyangiitis (GPA, formerly Wegener’s granulomatosis)
• Microscopic polyangiitis (MPA)
• Eosinophilic granulomatosis with polyangiitis (EGPA, formerly Churg–Strauss syndrome)

These diseases often present with non-specific symptoms:

• Sinusitis, nasal crusting or bleeding
• Cough, breathing difficulties
• Skin rashes
• Kidney dysfunction

ANCA testing helps confirm or support a diagnosis of these conditions, especially when combined with clinical findings and sometimes histopathology 1,3,4,5.

Disease Monitoring & Prognosis

Once a diagnosis is established, ANCA testing may be repeated to monitor disease activity, particularly in patients with renal involvement. Rising ANCA titers (amounts) can precede clinical relapse in some, but not all, patients 8,9,10.

• For renal (kidney) involvement: Serial ANCA measurements can help predict relapses.
• For non-renal disease: The value of repeat ANCA testing is less clear and remains controversial.

Aid in Disease Classification

ANCA testing not only diagnoses but also helps classify types of vasculitis:

• PR3-ANCA (C-ANCA pattern) is more common in GPA.
• MPO-ANCA (P-ANCA pattern) is more often seen in MPA and EGPA.
• Some forms, such as skin-limited polyarteritis nodosa, are typically ANCA-negative 2,4,6.

Exclusion of Other Conditions

Importantly, a negative ANCA test does not rule out AAV, as some patients can have the disease without detectable antibodies. Conversely, ANCA can be positive in other diseases, including infections (e.g., infective endocarditis) and drug-induced syndromes, so the test should always be interpreted in a clinical context 4,10.

Anca Test Details

ANCA testing encompasses several sophisticated laboratory techniques. The goal is to detect the presence of autoantibodies and, if present, identify their specific targets. Advances in technology have increased both the accuracy and reliability of these tests.

Table 2: ANCA Test Methods and Considerations

Test Methodologies

Indirect Immunofluorescence (IIF):

• The gold standard for screening.
• Patient serum is mixed with neutrophils fixed on a slide; if ANCA is present, it binds and emits a pattern when viewed under a fluorescence microscope.
• Two main patterns:
  • C-ANCA (Cytoplasmic): Associated mainly with proteinase 3 (PR3)
  • P-ANCA (Perinuclear): Associated mainly with myeloperoxidase (MPO)
• Other, less common patterns can occur, highlighting the need for interpretation by experienced personnel 1,3,4,5.

ELISA (Enzyme-Linked Immunosorbent Assay):

• Confirms and quantifies antibodies directed against specific neutrophil proteins (PR3 and MPO).
• First-generation (direct ELISA), second-generation (capture ELISA), and third-generation (anchor ELISA) versions exist, with newer generations offering higher sensitivity and specificity 3,6,7.

Emerging Technologies:

• Automated image analysis for IIF
• Bead-based multiplex assays for simultaneous detection of multiple antibodies
• These advances aim to standardize results and minimize human error 3,6,10.

Typical Testing Algorithm

• Initial screening: IIF performed on all serum samples.
• Confirmation: If positive or indeterminate, ELISA is performed for PR3 and MPO.
• Interpretation: IIF pattern and ELISA results are combined for highest accuracy 1,3,4,5,6.

Pitfalls and Limitations

• False Positives: Some infections, autoimmune diseases, and drugs can trigger ANCA positivity.
• False Negatives: Some patients with active vasculitis may have negative ANCA tests.
• Non-standardized reporting: Variability in lab techniques can affect results.
• Other antigens: Occasionally, ANCAs target other, less common neutrophil proteins (e.g., elastase), complicating interpretation 1,2,4,5,10.

When and How to Order the Test

• Order ANCA testing if you suspect small-vessel vasculitis or necrotizing arteritis based on clinical and laboratory features.
• Always interpret in context: A positive ANCA supports but does not confirm the diagnosis; clinical, laboratory, and sometimes tissue biopsy findings are necessary 4,10.

Anca Test Results & Follow-Up

Interpreting the results of ANCA testing can be challenging and must be done with a nuanced understanding of the clinical context. The test can provide critical information for diagnosis, guide therapy, and help monitor disease activity, but it is not infallible.

Table 3: ANCA Test Results and Clinical Actions

Interpreting Positive Results

• PR3-ANCA (C-ANCA pattern): Strongly associated with GPA. Not entirely exclusive, but supports this diagnosis in the right context 2,4,6.
• MPO-ANCA (P-ANCA pattern): More commonly found in MPA and EGPA. Also seen in some systemic PAN cases 2,4,6.
• Other antibodies: Rarely, ANCA may target other antigens, which may be relevant in atypical or unclassified vasculitis 2.

Negative Results: Not Always Exclusion

• A negative ANCA test does not exclude AAV. Up to 10-20% of patients with classic features may be ANCA-negative, particularly those with limited disease 4,10.
• Non-vasculitic conditions: ANCA may be absent or present at low, non-specific levels.

Serial Testing and Disease Monitoring

• Relapse prediction: Rising ANCA levels can precede clinical relapse, particularly in patients with renal involvement. However, this is not universally true, and not all relapses are accompanied by rising titers 8,9,10.
• Therapy guidance: Adjustments in therapy based solely on ANCA levels are not standard practice, except in experimental settings (e.g., rituximab-treated patients) 6,10.
• Routine monitoring: The value of serial testing is controversial; best considered in conjunction with clinical signs and symptoms 8,9,10.

Limitations and Special Situations

• False positives: Drug-induced ANCA (e.g., certain antibiotics, levamisole-adulterated cocaine) and infections (infective endocarditis) can trigger positive results 10.
• Interpretation pitfalls: Positive ANCA without clinical evidence of vasculitis should prompt careful re-evaluation. Decisions about immunosuppressive therapy should not be based on ANCA results alone 1,4.

Conclusion

ANCA testing revolutionized the diagnosis and management of small-vessel vasculitis, but it is not a stand-alone tool. Careful clinical correlation and understanding of test limitations are essential for optimal patient care.

Key Takeaways:

• ANCA testing is crucial for diagnosing, classifying, and sometimes monitoring ANCA-associated vasculitides, especially GPA, MPA, and EGPA 1,3,4,5,6,7.
• Combination of IIF and ELISA maximizes specificity and sensitivity; newer assays and technologies are improving accuracy 1,3,4,5,6,7,10.
• Positive results support but do not confirm vasculitis; negative results do not exclude it 4,10.
• Rising ANCA titers may predict relapse, particularly in patients with renal disease, but serial monitoring is not universally recommended 8,9,10.
• Always interpret ANCA results in the appropriate clinical context and never in isolation 1,4,5,10.

By understanding the strengths and limitations of ANCA testing, clinicians and patients alike can make more informed decisions in the challenging landscape of systemic vasculitis.