The following is from The University of Chicago Celiac Disease Center’s Guide for Followup Testing:
New guidelines on the diagnosis and treatment of celiac disease by the North American Society for Pediatric Gastroenterology, Hepatology and Nutrition state that tTG-IgA testing should be used for follow-up care. Interpreting this test result is straightforward—a celiac on the gluten-free diet should have a negative test. The numerical value of the test is not important.
The University of Chicago Celiac Disease Center recommends additional testing, because the tTG test can sometimes be inaccurate in people with autoimmune disorders like Type 1 diabetes and thyroid disease. In addition, the tTG can sometimes become negative before a celiac has actually experienced significant healing.
For these reasons, Anti-Gliadin Antibodies (AGA) are also important. There are two types that need to be run: AGA- IgA, and AGA-IgG. In this circumstance, the numerical values of the tests are very important. The numbers should be as close to zero as possible, indicating a minimal antibody response to gluten.
tTG-IgA: This test result should be negative. The numerical value of the test doesn’t matter as long as the result is negative.
Anti-gliadin IgA: This result should have a very low negative value. In this case, the numerical value does matter, because a high negative test result still indicates that a patient is eating gluten. A low negative indicates that the diet is working well.