The Test Sample
What is being tested?
Celiac disease is an autoimmune disorder characterized by an inappropriate immune response to gluten, a protein found in wheat, and related dietary proteins in rye and barley. Celiac disease antibody tests are a group of assays developed to help diagnose and monitor the disease and a few other gluten-sensitive conditions. These tests detect autoantibodies in the blood that the body produces as part of the immune response.
This immune response leads to inflammation of the small intestine and to damage and destruction of the villi that line the intestinal wall. The villi are projections, small tissue folds that increase the surface area of the intestine and allow nutrients, vitamins, minerals, fluids, and electrolytes to be absorbed into the body. When a susceptible person is exposed to gluten, the person's body produces autoantibodies that act against constituents of the intestinal villi. When villi are damaged or destroyed, the body is much less capable of absorbing food and begins to develop symptoms associated with malnutrition and malabsorption.
In the past, the only way to diagnose celiac disease was by examination of a tissue biopsy of the small intestine. While this microscopic evaluation is still considered the gold standard and is still used to confirm a diagnosis, the availability of less invasive blood tests to screen for celiac disease has reduced the number of biopsies needed.
Celiac disease blood tests measure the amount of particular autoantibodies in the blood. Tests that detect the IgA class (immunoglobulin A) and IgG class (immunoglobulin G) of the autoantibodies are available, but the types that measure IgA are more specific and are used almost exclusively. IgG and IgA are two of five classes of antibody proteins that the immune system produces in response to a perceived threat. IgA is the primary antibody present in gastrointestinal secretions.
IgG autoantibody tests may be ordered if a person has a deficiency in IgA. This happens about 2-3% of the time in people with celiac disease and can lead to false-negative test results.
Common tests for celiac disease include:
- Anti-tissue transglutaminase antibody (anti-tTG), IgA: detects antibodies to tissue transglutaminase, an enzyme that causes the crosslinking of certain proteins. Anti-tTG, IgA is the most sensitive and specific blood test for celiac disease. The IgG class of anti-tTG may be ordered for people who have a deficiency of IgA.
- Quantitative immunoglobulin A (IgA): measures the total level of IgA in the blood to determine if someone is deficient in the IgA class of antibodies
- Deamidated gliadin peptide (DGP) antibodies, IgA: detects anti-DGP IgA antibodies; like anti-tTG, the IgG class may be performed for a person with an IgA deficiency.
Other tests less commonly performed include:
- Anti-endomysial antibodies (EMA), IgA class: detects antibodies to endomysium, the thin connective tissue layer that covers individual muscle fibers
- Anti-reticulin antibodies (ARA), IgA class: not as specific or sensitive as the other autoantibodies
For additional details on these tests, see the "How is it used?" section.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Follow your health practitioner's instructions. For diagnosis, an individual should continue to eat foods that contain gluten for a time period, such as several weeks, prior to testing. For monitoring someone who has already been diagnosed, no preparation is necessary.