Two types of testing are used to diagnose celiac disease.
The first is a blood test to measure whether there are antibodies (immunoglobulins) to injury, caused by gluten, are in the blood. A person with celiac disease has higher than normal levels of these antibodies in their blood. Antibodies are proteins produced by the immune system in response to substances that the body perceives to be foreign or threatening. For people with celiac disease, the gluten protein is the foreign substance that will cause production of antibodies.
To screen for celiac disease using a blood test, physicians will usually measure levels of:
- Immunoglobulin A (IgA)
- IgA anti-tissue transglutaminase antibodies (anti-tTG IgA)
- IgA anti-endomysial antibodies (EMA IgA)
The screening blood antibody tests are very helpful, but false positive and false negative results can occur. If a person has a positive screening blood antibody test, an experienced healthcare provider will recommend performing a procedure called an endoscopy with a small intestinal biopsy. This procedure is safe, common and quick and should always be performed by an experienced pediatric gastroenterologist.
Almost all children will have the endoscopy and biopsies done under anesthesia, which will guarantee that they will neither feel nor remember the procedure. The endoscope is a thin, flexible tube with a built-in camera system that allows the doctor to see inside the body. The endoscope slides over the tongue, down the esophagus and into the stomach and then the intestine. Using the endoscope, tiny pieces of the intestine are removed to check for damage.
Sometimes a genetic test is done for celiac disease. This test will determine the presence of two genes, HLA-DQ2 and DQ8, which are present in most people with celiac disease and absent in most people who do not have the disease. However, the genetic test cannot give a final answer about whether a person has celiac disease. A positive test is found in about 30-40% of the US population but only about 1% of the population will develop the disease. A negative test makes it very unlikely that a person has celiac disease but does not completely rule it out.
A person must be eating gluten to be tested for celiac disease. If a person stops eating foods with gluten before being tested, the body will stop producing the antibodies that are associated with the injury from gluten. The results of either the screening blood tests or endoscopy with biopsies may be falsely negative.