ANCA/MPO/PR3 Antibodies

An ANCA test is requested in order to evaluate someone who has symptoms that may be due to autoimmune vasculitis. It may be requested 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, complement levels to assess inflammation and creatinine to evaluate kidney function. Sometimes an ANCA test may be requested 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 requested when a person is suspected of having a systemic autoimmune vasculitis, especially when symptoms suggest Granulomatosis with polyangitis (GPA ) 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 small 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 most 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 after a sample has been tested and is positive for MPO or PR3. 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. 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 many cases of ulcerative colitis as well as some patients with Crohn's disease and rheumatoid arthritis.

The majority of laboratories will follow the new international guidance and the ANCA IIF will only be performed on samples already screened and known to be positive for MPO or PR3;

• A positive test for PR3 antibodies and a positive c-ANCA or p-ANCA are seen in more than 80% of patients with active GPA
• A positive test for MPO antibodies and a positive p-ANCA are consistent with microscopic polyangitis, glomerulonephritis, Churg-Strauss syndrome, and Goodpasture's syndrome. MPO and p-ANCA may also be present non specifically 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 will continue to have it.