Diagnostic Testing for Celiac Disease
What is the diagnostic test?
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The testing has changed since I was trained, when we looked for celiac disease with antigliadin antibodies. Then, we looked for antireticulin antibodies, followed by antiendomysial antibodies. Now the antiendomysial antibody or the IgA anti-tissue transglutaminase (TTG) tests are in favor.
This is where we are now. This is an IgA antibody test. If a patient is IgA deficient, this isn't a very good test. But if a patient has normal IgA levels, the test has 95% or higher sensitivity and specificity. This is a very significant advance as far as diagnostic testing strategies for celiac disease.
There are other diagnostic tests. The HLA-DQ2 and -DQ8 genotypes are abnormally in virtually all patients with celiac disease. The HLA-DQ2 test picks up 95%, and the HLA-DQ8 another 5% of patients. The negative predictive value for both tests is higher than 99% -- as close as we can get to being perfect.
The problem with this test -- and the reason we don't test everybody with this genetic test -- is that 25%-30% of the white population of the United States will test positive, but they don't have celiac disease. They test positive for the predisposition for celiac disease, so this isn't the starting strategy for diagnostic testing for celiac disease.
Testing is an issue in patients who are IgA deficient. Most of the test strategies will check an IgA level. You have to move on to deamidated gluten peptide (DGP), an IgG antibody test. Or you can test for the IgG TTG level. These tests would be helpful in patients who are IgA deficient.
If the patient is a child younger than 2 years, these are the preferred tests because their IgA levels may not be proficient. We always default to the TTG in children younger than 2 years, or couple it with DGP levels. The testing strategy in young children is different. If you see pediatric patients, you need to be aware of that.
It is important to realize that when you do these diagnostic tests, many patients have been on the Internet. Many will come in asking to be tested for celiac disease, but they have already been on a gluten-free diet, so they have set themselves up for a negative test result. That eliminates the sensitivity of detection for celiac disease. Within weeks on a gluten-free diet, the antibody response is diminished. If you test patients who have been on a gluten-free diet for 6-12 months, approximately 80% of people will lose their antibody response. At 5 years, the number is in excess of 90%.
Therefore, when you are considering celiac disease, take a good dietary history before you embark on serologic testing. The results can be misleading. The same is true for biopsies, which will also change on a gluten-free diet.