An antineutrophil cytoplasmic antibodies (ANCA) test is a blood test that checks for proteins your immune system makes to fight germs. This test can help your doctor distinguish ulcerative colitis from Crohn's disease. It can also help your doctor diagnose autoimmune diseases like vasculitis, a blood vessel disease.
ANCAs are a type of autoantibody. Unlike the antibodies your immune system makes to fight germs, autoantibodies attack and damage your healthy tissues. Having autoantibodies in your blood is a sign that you could have an autoimmune disease.
ANCAs attack neutrophils, which are the white blood cells your immune system sends out to fight infections.
What Is It Used For?
The ANCA test checks for the two main types of ANCAs, each of which targets a different protein inside white blood cells:
- pANCA targets a protein called myeloperoxidase (MPO)
- cANCA targets a protein called proteinase 3 (PR3)
ANCA tests come in two types:
Indirect immunofluorescence (IIF). This test mixes your blood sample with neutrophils on a slide. A special stain is added to highlight the autoantibodies. If your blood contains ANCAs, they will attach to the neutrophils and form a pattern that can be seen under a microscope. pANCA and cANCA each create a different pattern.
Enzyme-linked immunosorbent assay (ELISA). This test checks your blood for antibodies to MPO and PR3. An ELISA test shows whether you have pANCA or cANCA.
When Should You Get This Test?
One reason to get an ANCA test is if your doctor isn't sure whether you have Crohn's disease or ulcerative colitis. The symptoms of these two types of inflammatory bowel disease (IBD) are often similar, making them hard to tell apart.
Doctors diagnose IBD with a combination of tests, including:
- Blood tests
- Physical exams
- Esophagogastroduodenoscopy (EGD) – a test that places a scope into your upper GI tract to look for changes in your esophagus, stomach, and the top of your small intestine
- Colonoscopy – a test that places a scope into your lower GI tract to look for changes in your large intestine and rectum
To make the diagnosis of ulcerative colitis, your doctor will ask if you have had chronic diarrhea for more than 4 weeks and will check to see if there is active inflammation and chronic changes on biopsy done during colonoscopy.
Sometimes these tests can't confirm which type of IBD you have. An ANCA test is a way for your doctor to help figure out whether you have ulcerative colitis or Crohn's disease when the diagnosis isn't clear.
Up to 80 percent of people with ulcerative colitis test positive for pANCA. And the PR3 protein is much more common in children with ulcerative colitis than in those with Crohn's disease.
The ANCA test can also help predict whether you'll respond to IBD medicines called tumor necrosis factor (TNF) inhibitors. TNF inhibitors block a protein your immune system makes that causes inflammation in your intestine.
ANCA testing is more commonly used to diagnose vasculitis, a group of autoimmune disorders that affect small blood vessels. In vasculitis, ANCAs attach to neutrophils and cause them to attack the blood vessels. This attack makes the blood vessels swell up.
Three types of vasculitis are linked to ANCA:
- Eosinophilic granulomatosis with polyangiitis (EGPA) inflames blood vessels in the heart and kidneys.
- Granulomatosis with polyangiitis (also called Wegener granulomatosis) affects blood vessels in the lungs and kidneys.
- Microscopic polyangiitis (MPA) affects blood vessels in the kidneys, lungs, skin, and joints.
Vasculitis causes different symptoms depending on which blood vessels it affects. For example, it causes a rash when it inflames blood vessels in the skin. And there may be foam or blood in the urine if it attacks the kidneys.
The ANCA test can be helpful for diagnosing vasculitis if you have symptoms like these:
- Weight loss
- Muscle or joint aches
- Vision or hearing loss
- Skin rashes or sores
- Foamy urine or blood in the urine
The ANCA test can also show if the treatment you take for vasculitis is working. And your doctor can use it to predict whether your symptoms might come back after treatment, which is called a relapse.
How Is the Test Done?
Your doctor will let you know what to do before you have this test. No special preparation is needed for an ANCA test, but If you're having another blood test with it, you may need to fast for 8 to 12 hours beforehand.
You'll get this test at a doctor's office, clinic, or lab. A health care provider will take a sample of blood from a vein in your arm. Then the sample will get sent out to a lab.
You may have a small bruise on your skin in the area where the test was done.
What Do Your Results Mean?
It will take a few days to get your results. If the test is positive, you may have an autoimmune vasculitis. The type of ANCA may help determine which autoimmune vasculitis you have. You may have both types of ANCA.
If you are having symptoms of IBD, a positive result may mean that you are more likely to have ulcerative colitis rather than Crohn's disease. Up to 80% of the people with ulcerative colitis will have a positive ANCA test.
Your doctor will explain the meaning of your test results to you. Then you can discuss next steps, which might include more tests or treatments if you have a positive result.