An ANCA test is ordered to evaluate someone who has symptoms that may be due to autoimmune vasculitis. It may be ordered by itself or along with two other tests that are used to distinguish between different autoimmune disorders. These include:

• MPO antibodies
• PR3 antibodies

Additional tests that may be performed to aid in a diagnosis include ESR (erythrocyte sedimentation rate) to check for inflammation, FBC (full blood count) to measure and evaluate white and red blood cells, and urinalysis, urea, and creatinine to evaluate kidney function.

Sometimes an ANCA test may be ordered to monitor the response to therapy, follow the course of the disease, and/or to detect a relapse.

An ANCA test and/or tests for MPO and PR3 are ordered when a person is suspected of having a systemic autoimmune vasculitis, especially when symptoms suggest GPA (formerly Wegener's granulomatosis) or microscopic polyangiitis. Early in the disease, symptoms may be vague or nonspecific, such as fever, fatigue, weight loss, muscle and/or joint aches, and night sweats.

As the disease progresses, damage to blood vessels throughout the body may cause signs and symptoms associated with complications involving various tissues and organs. A few examples include:

• Eyes — red, itchy eyes or "pink eye" (conjunctivitis); problems with sight (blurry vision, loss of vision)
• Ears — hearing loss
• Nose — runny nose or other upper respiratory symptoms that do not go away
• Skin — rashes and/or granulomas
• Lungs — cough and/or difficulty breathing
• Kidneys — abnormal amounts of protein in the urine

Some laboratories will perform all three tests, ANCA, MPO and PR3, as a panel while others will perform MPO and PR3 only if an initial ANCA test is positive.

Testing may also be performed periodically to monitor a person who has been diagnosed with an autoimmune vasculitis.

Results of ANCA tests must be interpreted carefully and take several factors into account. A doctor will consider clinical signs and symptoms and results of other types of tests such as imaging studies when determining a diagnosis.

Positive ANCA, PR3, and/or MPO tests help to support a diagnosis of systemic autoimmune vasculitis and to distinguish between different types. However, to confirm a diagnosis, a biopsy of an affected site is often required. Negative tests results make it less likely that a person's symptoms are due to an autoimmune vasculitis.

Most often, ANCA tests are performed using indirect immunofluorescence microscopy. Serum samples are mixed with neutrophils to allow autoantibodies that may be present to react with the cells. The sample is put on a slide and treated with a fluorescent stain. The slide is then examined under a microscope and the resulting pattern noted.

Several different ANCA patterns may be seen:

• Perinuclear (p-ANCA) – most of the fluorescence occurs near the nucleus. About 90% of samples with a p-ANCA pattern will have MPO antibodies.
• Cytoplasmic (c-ANCA) – the fluorescence occurs throughout the cytoplasm of the cell and is accentuated between the nuclei. About 85% of samples with a c-ANCA pattern will have PR3 antibodies.
• Negative ANCA — very little or no fluorescence
• Atypical p-ANCA — seen in most cases of ulcerative colitis as well as some patients with Crohn's disease. Atypical or flat C-ANCA-seen in some cases of bacterial endocarditis, rheumatoid arthritis, other auto-immune disease and in treated cases of GPA. 
• It is not uncommon for patients who have vasculitis to have slightly atypical ANCA patterns due to various laboratory and patient factors. In some cases it will be important for your doctor to discuss results with the reporting immunologist at the pathology laboratory.

If an ANCA test result is positive, then an additional test is performed to determine the amount of antibody present. This is called a  titre. To determine the titre, a serum sample is diluted in steps and each dilution is tested for the presence of the antibody. The greatest dilution at which the antibody can be detected is the titre. For example, if a serum tests positive after being diluted 64-fold, the titre is 1:64. The higher the titre, the more antibody is present in the blood.

In addition, tests that are specific for antibodies to MPO and PR3 may be performed:

• A positive test for PR3 antibodies and a positive c-ANCA or p-ANCA are seen in more than 90% of patients with active Granulomatosis with Polyangiitis (Wegener's granulomatosis).
• A positive test for MPO antibodies and a positive p-ANCA are consistent with microscopic polyangiitis, glomerulonephritis, eGPA (Churg-Strauss syndrome), and Goodpasture's syndrome. MPO and p-ANCA may also be present in other autoimmune disorders, such as systemic lupus erythematosus, rheumatoid arthritis, and Sjögren syndrome.

The following table shows results that may be seen in some vasculitis conditions.

ANCA levels can change over time and may sometimes be used in a general way to monitor disease activity and/or response to therapy.

In most cases, a biopsy of an affected blood vessel is necessary to confirm a diagnosis of autoimmune vasculitis.

Levels may fluctuate but once a person has developed an autoantibody, they are likely to continue to be able to produce it, although it may not be readily detectable.

ANCA, MPO and PR3 tests require specialised equipment. Your sample will need to be sent to a clinical laboratory that performs these tests.

ANCA testing is only necessary when a person has symptoms that suggest systemic vasculitis. Most people will never need to have an ANCA test performed.

Yes, there are several. Some of these include rheumatoid arthritis, inflammatory bowel disease, certain drugs, and infections involving the heart (endocarditis) or the respiratory system.

There is quite a long list of autoimmune disorders that have been identified (see Related Pages tab). They are organised, with some overlap, by the size of the blood vessels that they affect - small, medium or large.