According to my information, you don't need to fast at all for a blood test for Celiac Disease. If you fast the doctors will not get the accurate results. Just avoid a fatty meal prior to the test.
This is the type of question you need to ask people at the facility who are going to render the test. Facilities have different equipment which would require different preps for tests. If is important that you get this information from them so the test will not have to be postponed or repeated which can be timely and costly.