Celiac disease is an autoimmune condition triggered by the consumption of gluten, a protein found in wheat, barley, and rye. This disease causes damage to the small intestine, impairing its ability to absorb nutrients. Blood tests, particularly those that measure Immunoglobulin A (IgA) antibodies, are often the initial step in identifying individuals who might have celiac disease.
The Role of IgA in Celiac Disease
Immunoglobulin A (IgA) is a type of antibody, a protein that identifies and neutralizes foreign substances like bacteria and viruses. IgA is particularly abundant in the mucous membranes lining the digestive, respiratory, and urogenital tracts, for local immunity. In individuals with celiac disease, the immune system mistakenly produces IgA antibodies that target the body’s own tissues.
Specifically, these IgA antibodies often target an enzyme called tissue transglutaminase (tTG), which is normally involved in repairing and healing cells in the body. When gluten is consumed by someone with celiac disease, the immune system produces IgA antibodies against this enzyme, leading to inflammation and damage in the small intestine.
Specific IgA Antibody Tests
Several IgA-based blood tests are commonly used to screen for celiac disease. The most widely used test is the tissue transglutaminase IgA (tTG-IgA) antibody test. This test is considered highly sensitive and specific for celiac disease in most children and adults.
Another important test is the Endomysial Antibody IgA (EMA-IgA). This test is known for its high specificity, though it may be slightly less sensitive than tTG-IgA. EMA-IgA is often used as a confirmatory test following a positive tTG-IgA result. Deamidated Gliadin Peptide IgA (DGP-IgA) is also used, particularly in cases where tTG-IgA might be less reliable, such as in very young children or in individuals with an IgA deficiency. DGP-IgA tests measure antibodies against deamidated gliadin, a modified component of gluten.
Interpreting IgA Test Levels
Laboratories provide reference ranges for IgA antibody tests, and levels significantly above the upper limit of these normal ranges suggest a strong possibility of celiac disease. An elevated IgA antibody level suggests celiac disease, though it is not a definitive diagnosis. A negative result means these antibodies were not detected.
Measuring total serum IgA is an important step alongside specific antibody tests. This is because some individuals have selective IgA deficiency, a genetic condition causing very low IgA levels. This can lead to false-negative results for IgA-based celiac tests. Selective IgA deficiency is more common in people with celiac disease than in the general population, affecting about 2-3% of those with the condition. When IgA deficiency is detected, doctors may order IgG-based antibody tests, such as tTG-IgG or DGP-IgG, to screen for celiac disease.
Beyond IgA: The Full Diagnostic Picture
While IgA antibody tests are an important first step in identifying celiac disease, they serve as a screening tool, not a definitive diagnosis. An intestinal biopsy of the small bowel is generally considered the gold standard for confirming celiac disease. This procedure involves an endoscopy where small tissue samples are taken from the small intestine to check for characteristic damage to the villi, which are tiny finger-like projections responsible for nutrient absorption.
Genetic testing for human leukocyte antigens (HLA-DQ2 and HLA-DQ8) can also provide important information. The presence of these genes is necessary for celiac disease to develop, but their presence alone does not mean an individual has or will develop the disease. The absence of both HLA-DQ2 and HLA-DQ8, however, can effectively rule out celiac disease. It is important that individuals continue consuming gluten throughout the blood testing and biopsy process to ensure accurate results. Discontinuing gluten before testing can lead to inaccurate or false-negative outcomes. Interpretation of all test results and subsequent diagnostic steps should always be performed in consultation with a healthcare professional.