ANCA (Antineutrophil Cytoplasmic Antibodies) are a group of autoantibodies directed against proteins found in the cytoplasm of neutrophils, a type of white blood cell. ANCA testing is primarily used to help diagnose certain autoimmune conditions, particularly vasculitis—inflammation of the blood vessels. The presence of ANCA can indicate that the immune system is mistakenly attacking blood vessels, which can lead to damage in various organs.

Why is ANCA Testing Done?

ANCA testing is often performed when there is suspicion of vasculitis or other autoimmune diseases, especially if a person presents with:

• Unexplained fever
• Fatigue
• Joint pain
• Weight loss
• Organ-specific symptoms (e.g., kidney problems, lung issues, skin rashes)

It can also be used to monitor disease activity in patients already diagnosed with ANCA-associated vasculitis.

Types of ANCA Testing

ANCA testing is typically conducted using two main methods:

1. Indirect Immunofluorescence (IFA):

• This method detects the presence of ANCA antibodies by looking at the pattern of fluorescence in neutrophil cells. There are two common patterns:
• Cytoplasmic pattern (c-ANCA): The antibodies target proteins in the cytoplasm of neutrophils.
• Perinuclear pattern (p-ANCA): The antibodies target proteins around the nucleus of neutrophils.

1. Enzyme-Linked Immunosorbent Assay (ELISA):

• This method specifically detects certain ANCA types by measuring the amount of antibodies present against particular neutrophil proteins (e.g., proteinase 3 (PR3) and myeloperoxidase (MPO)).

What Are c-ANCA and p-ANCA?

• c-ANCA (Cytoplasmic ANCA):
• Associated with Wegener's Granulomatosis (Granulomatosis with Polyangiitis), a form of granulomatosis vasculitis that primarily affects the respiratory tract and kidneys. The PR3 protein is often the target of c-ANCA antibodies.
• p-ANCA (Perinuclear ANCA):
• Associated with Microscopic Polyangiitis and Churg-Strauss Syndrome. The antibodies are directed against myeloperoxidase (MPO), a protein found in neutrophil granules.
• p-ANCA can also be seen in other autoimmune diseases like ulcerative colitis or rheumatoid arthritis.

Conditions Associated with Positive ANCA Results

1. Granulomatosis with Polyangiitis (GPA):

• Formerly known as Wegener’s granulomatosis, this is most commonly associated with c-ANCA (targeting PR3).
• Affects the respiratory tract (sinusitis, nosebleeds, coughing) and kidneys (glomerulonephritis).

1. Microscopic Polyangiitis (MPA):

• Strongly associated with p-ANCA (targeting MPO), though it can also sometimes show c-ANCA.
• Affects the kidneys, lungs, and sometimes the skin.

1. Churg-Strauss Syndrome:

• Associated with p-ANCA and involves asthma, eosinophilia (elevated eosinophils), and vasculitis, primarily affecting the lungs and skin.

1. Other Vasculitides:

• Polyarteritis Nodosa, Henoch-Schönlein Purpura, and other forms of small-vessel vasculitis can sometimes be associated with ANCA.

1. Ulcerative Colitis and Crohn’s Disease:

• Though not classic ANCA-associated diseases, some individuals with inflammatory bowel disease (IBD) can have p-ANCA positivity, particularly in ulcerative colitis.

1. Rheumatoid Arthritis:

• Some individuals with RA may show a p-ANCA pattern, though it's less specific than in vasculitis.

Interpreting ANCA Test Results

• Positive c-ANCA (PR3-ANCA):
• Strongly suggestive of Granulomatosis with Polyangiitis.
• Positive p-ANCA (MPO-ANCA):
• Associated with Microscopic Polyangiitis or Churg-Strauss Syndrome.
• Low Titers:
• Low titers of either c-ANCA or p-ANCA can be seen in a variety of conditions, including other autoimmune diseases, infections, or even in healthy individuals, so clinical context is important.
• Negative ANCA:
• A negative ANCA test does not rule out vasculitis, as some forms of vasculitis do not produce detectable ANCA antibodies.

Limitations of ANCA Testing

• False Positives:
• Some individuals without vasculitis can have positive ANCA results. This can happen in conditions like inflammatory bowel disease or rheumatoid arthritis or even in healthy individuals.
• False Negatives:
• Not all patients with vasculitis will have a positive ANCA test. A negative result does not exclude the possibility of vasculitis.
• Other Factors:
• Medications (like propylthiouracil and methimazole) and infections can also cause ANCA positivity.

Follow-up and Further Testing

If ANCA testing suggests the presence of vasculitis or another autoimmune condition, further diagnostic steps may include:

1. Biopsy:

• A biopsy of affected tissue (such as the kidneys, lungs, or skin) may be necessary to confirm the diagnosis of vasculitis.

1. Other Laboratory Tests:

• Additional tests for specific antibodies (e.g., anti-dsDNA for lupus) or other vasculitis markers (e.g., CRP, ESR) may be ordered.

1. Imaging Studies:

• Imaging of the affected organs, like the kidneys, lungs, or sinuses, may be used to assess damage or disease progression.

Key Takeaways

• ANCA Testing helps diagnose and monitor ANCA-associated vasculitis, particularly Granulomatosis with Polyangiitis and Microscopic Polyangiitis.
• c-ANCA is most often associated with PR3 and Wegener’s granulomatosis, while p-ANCA is linked to MPO and Microscopic Polyangiitis.
• Positive ANCA must be interpreted in the context of clinical symptoms and other diagnostic tests, as a positive result can occur in other conditions, or even in healthy individuals.