An ANCA screen with MPO and PR3 is a blood test panel that checks for specific antibodies linked to autoimmune vasculitis, a group of conditions where the immune system attacks small blood vessels. The panel has two parts: a screening test that detects antibodies called antineutrophil cytoplasmic antibodies (ANCA), and confirmatory tests that measure antibodies targeting two specific proteins, myeloperoxidase (MPO) and proteinase 3 (PR3). Together, these results help identify whether vasculitis is present and which type you may have.
What ANCA, MPO, and PR3 Mean
ANCA stands for antineutrophil cytoplasmic antibodies. These are immune proteins that mistakenly attack neutrophils, a type of white blood cell that normally fights infection. When these antibodies latch onto neutrophils, they trigger inflammation in the walls of small blood vessels throughout the body. That inflammation is vasculitis, and it can damage the kidneys, lungs, sinuses, skin, and nervous system.
MPO and PR3 are the two proteins inside neutrophils that ANCA antibodies target. MPO (myeloperoxidase) is an enzyme neutrophils use to kill bacteria. PR3 (proteinase 3) is another enzyme involved in immune defense. When your immune system produces antibodies against one of these proteins, the specific target helps doctors determine which form of vasculitis you have, because each antibody type is strongly associated with a different disease pattern.
How the Test Works
The panel typically involves two layers of testing done on a single blood draw. The initial screen uses a technique called indirect immunofluorescence (IIF), where your blood serum is placed on a slide containing fixed human neutrophils. If ANCA antibodies are present, they bind to the neutrophils and glow under a fluorescent microscope. The pattern of that glow matters: a cytoplasmic pattern (called c-ANCA) lights up the body of the cell and usually indicates PR3 antibodies, while a perinuclear pattern (called p-ANCA) glows around the nucleus and typically corresponds to MPO antibodies.
The second layer uses a more targeted method to measure the exact levels of anti-MPO and anti-PR3 antibodies in your blood. This is done through an immunoassay where your serum is exposed to purified MPO and PR3 proteins on a test plate. If antibodies are present, they bind to these proteins, and the resulting color change is measured to give a numerical value. This step confirms which protein your antibodies are targeting and how much is circulating.
Reading Your Results
Reference ranges can vary between labs, but a widely used scale works like this: for both MPO and PR3 antibodies, a level of 19 AU/mL or less is negative, 20 to 25 AU/mL is equivocal (borderline), and 26 AU/mL or greater is positive. The IIF screening portion is reported as a pattern (c-ANCA, p-ANCA, or none detected) along with a titer, where less than 1:20 is considered negative.
A positive result does not automatically mean you have vasculitis. It means these antibodies are present in your blood, which raises suspicion and typically leads to further evaluation, including imaging, tissue biopsy, or additional bloodwork. An equivocal result may prompt repeat testing after a few weeks.
Conditions Linked to Each Antibody
The distinction between MPO and PR3 antibodies is clinically meaningful because the two are associated with different diseases and different patterns of organ involvement.
PR3 antibodies are strongly linked to granulomatosis with polyangiitis (GPA, formerly called Wegener’s granulomatosis). PR3 testing picks up about 74% of GPA cases. This form of vasculitis tends to affect more organs overall, with particular involvement of the ears, nose, throat, and eyes. Lung disease in PR3-associated vasculitis often produces cavitating lesions (holes in lung tissue) and lung hemorrhage. People with PR3-positive vasculitis also have a higher relapse rate after treatment.
MPO antibodies are more closely tied to microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis (EGPA, formerly Churg-Strauss syndrome). MPO testing detects roughly 73% of MPA cases. MPO-associated vasculitis more often stays confined to the kidneys and tends to cause scarring-type damage in the lungs rather than cavities. It relapses less frequently than PR3-associated disease, though kidney outcomes and cardiovascular events can be worse over time.
Both antibody types carry specificities around 97%, meaning a positive result for either one rarely shows up in people without vasculitis. That high specificity is what makes the panel so useful for diagnosis.
Why Your Doctor Ordered This Test
This panel is typically ordered when someone has a combination of symptoms that suggest small blood vessel inflammation. Common triggers for ordering the test include unexplained kidney problems (blood or protein in the urine, rising creatinine), persistent sinusitis or nasal crusting that doesn’t respond to normal treatment, coughing up blood, unexplained nerve pain or numbness, skin rashes with purplish spots, or lung abnormalities on imaging that don’t fit a clear diagnosis. The combination of kidney and lung symptoms together is an especially strong prompt for ANCA testing.
The test is also used to monitor people already diagnosed with ANCA-associated vasculitis. Rising antibody levels sometimes signal a flare before symptoms appear, though this relationship isn’t perfectly reliable and rising levels alone don’t always require a change in treatment.
What Can Cause a False Positive
ANCA levels, particularly MPO antibodies, can be elevated in situations that have nothing to do with vasculitis. Rheumatoid arthritis is one of the more common causes of a false-positive MPO result. Certain infections can also temporarily raise MPO-ANCA levels. In documented cases, MPO levels returned to normal after the infection was treated, confirming the elevation was reactive rather than a sign of underlying vasculitis.
Other autoimmune conditions, inflammatory bowel disease, and some medications can produce positive ANCA results as well. This is why a positive screen almost always requires clinical correlation: your doctor will weigh the lab result against your symptoms, physical exam, and other test findings before reaching a diagnosis.
Geographic and Demographic Patterns
PR3-associated vasculitis is more common in northwestern Europe and North America, while MPO-associated vasculitis is more prevalent in southern Europe, Asia, and the Pacific region. PR3-positive disease has a slightly higher male-to-female ratio compared to MPO-positive disease. Age at onset is similar for both types. These patterns reflect underlying genetic differences in susceptibility, with different immune system genes predisposing people to one antibody type over the other.