<<Disclaimer: Verify this information before applying it to your situation.>> I received three responses to my post. However, I did not receive an answer as to whether the tissue Transglutaminase test would indicate celiac disease if the person being tested was on the celiac diet (avoiding gluten). My guess is that being on the diet would result in the person being tested to be just like a non-celiac, and therefore the test would probably be negative. However, I do not have any facts.