Final answer:
The definitive test for diagnosing celiac disease is the Endomysial antibody test.
Step-by-step explanation:
The definitive test for diagnosing celiac disease is the Endomysial antibody test. This test measures the presence of autoantibodies to endomysium, a connective tissue surrounding muscle fibers. These antibodies are produced in response to gluten, the protein component found in wheat, barley, rye, and some other grains. The Endomysial antibody test is typically performed after serological tests, which detect IgA antibodies to gluten components and the transglutaminase enzyme.