An ANCA blood test checks for antineutrophil cytoplasmic antibodies, a type of autoantibody that mistakenly attacks enzymes inside your own white blood cells. When these antibodies are present, they can trigger inflammation in small blood vessels throughout the body, a group of conditions collectively called ANCA-associated vasculitis. The test is most commonly ordered when a doctor suspects this type of vasculitis based on symptoms like unexplained kidney problems, lung issues, or widespread inflammation that doesn’t have an obvious cause.
What ANCA Antibodies Actually Do
Neutrophils are white blood cells that normally fight infection. They contain powerful enzymes that help destroy bacteria and other invaders. In some people, the immune system produces antibodies that target those enzymes while they’re still inside the neutrophils. When these ANCA antibodies bind to neutrophil enzymes, they activate the white blood cells in the wrong place and at the wrong time, causing them to attack the walls of small blood vessels instead of fighting infections.
This misdirected immune response leads to inflammation and damage in the blood vessels of the kidneys, lungs, sinuses, skin, and other organs. Left untreated, it can cause serious organ damage. The ANCA test helps identify whether this autoimmune process is behind a patient’s symptoms.
The Two Types of ANCA
The test doesn’t just look for ANCA in general. It distinguishes between two main types, each targeting a different enzyme and associated with different conditions.
PR3-ANCA (sometimes called c-ANCA): These antibodies target an enzyme called proteinase 3. They are strongly linked to granulomatosis with polyangiitis (GPA), formerly known as Wegener’s granulomatosis. This condition typically affects the sinuses, lungs, and kidneys. In one study, over 92% of patients with PR3 antibodies showed the c-ANCA staining pattern under the microscope.
MPO-ANCA (sometimes called p-ANCA): These target an enzyme called myeloperoxidase. They’re more closely associated with microscopic polyangiitis (MPA), which primarily affects the kidneys and lungs, and with eosinophilic granulomatosis with polyangiitis (EGPA), previously called Churg-Strauss syndrome. Certain drug-induced forms of vasculitis also fall into this category.
Knowing which type of ANCA is present helps narrow down the specific diagnosis, since these conditions require different monitoring and treatment approaches.
How the Test Is Done
The ANCA test is a standard blood draw. No fasting or special preparation is typically required, though your doctor may order it alongside other blood tests that do need preparation. A sample is taken from a vein in your arm, and the process takes only a few minutes.
In the lab, the current recommended approach is to screen the blood directly for PR3 and MPO antibodies using an immunoassay. This has largely replaced an older method called indirect immunofluorescence, where lab technicians looked at staining patterns under a microscope. A 2017 international consensus confirmed that going straight to the specific antibody test provides better sensitivity without needing the fluorescence step first. If ANCA antibodies are found, additional testing may measure how much antibody is present in your blood, reported as a titer level.
What the Results Mean
A negative result means no ANCA antibodies were detected. This makes autoimmune vasculitis unlikely as the cause of your symptoms, though it doesn’t completely rule it out in rare cases.
A positive result means ANCA antibodies are present, and the report will specify whether they are PR3 or MPO type. This points toward autoimmune vasculitis, but a positive ANCA alone is not a final diagnosis. The 2022 ACR/EULAR classification criteria, developed from the largest vasculitis study ever conducted, now give significant weight to ANCA results alongside modern imaging. Still, doctors typically combine the ANCA result with imaging studies, urine tests, and often a tissue biopsy to confirm exactly what’s happening.
How Accurate Is the Test?
The accuracy depends on which antibody is being measured. PR3-ANCA testing performs very well: at standard cutoff levels, it has a sensitivity of 99.9% and a specificity of 92% for detecting ANCA-associated vasculitis. That means it catches nearly all true cases and rarely flags someone who doesn’t have the condition. A positive PR3 result is particularly helpful when GPA is suspected.
MPO-ANCA testing is somewhat less precise, with a sensitivity of 90% and a specificity of 55%. The lower specificity means a positive MPO result has a higher chance of being a false alarm, which is why doctors rely on additional tests before making a diagnosis.
What Else Can Cause a Positive Result
ANCA antibodies can sometimes appear in people who don’t have vasculitis at all. Autoimmune conditions like rheumatoid arthritis can nonspecifically elevate ANCA levels, particularly MPO-ANCA. Certain infections can do the same. In one documented case, a patient with rheumatoid arthritis tested positive for MPO-ANCA due to a lung infection, not vasculitis, which required careful differentiation before the right treatment could be chosen.
This is why a positive ANCA result is always interpreted in context. Your symptoms, organ involvement, imaging findings, and sometimes biopsy results all factor into whether the antibodies represent true vasculitis or an incidental finding from another condition.
Conditions the Test Helps Diagnose
ANCA-associated vasculitis is the most common type of small-vessel vasculitis. The specific conditions it encompasses include:
- Granulomatosis with polyangiitis (GPA): Affects the sinuses, lungs, and kidneys. Strongly linked to PR3-ANCA.
- Microscopic polyangiitis (MPA): Primarily affects the kidneys and lungs. More often linked to MPO-ANCA.
- Eosinophilic granulomatosis with polyangiitis (EGPA): Involves asthma, elevated eosinophils, and vasculitis affecting multiple organs. Associated with MPO-ANCA.
- Drug-induced vasculitis: Some medications can trigger ANCA production and vasculitis symptoms that resolve when the drug is stopped.
All of these conditions involve inflammation in small blood vessels, but they differ in which organs they target, how they progress, and how they respond to treatment. The ANCA type, combined with clinical findings, helps guide doctors toward the right diagnosis and treatment plan.