The Tissue Transglutaminase IgA (tTG-IgA) test is used primarily to help diagnose celiac disease, an autoimmune disorder where the ingestion of gluten leads to damage in the small intestine. The test measures the level of IgA antibodies against tissue transglutaminase, an enzyme that plays a role in the immune response in celiac disease. It is one of the most sensitive and specific tests for celiac disease.