Celiac disease blood tests are a primary screening tool when the condition is suspected. These tests detect specific antibodies the body produces in response to gluten, a protein found in wheat, barley, and rye. When someone with celiac disease consumes gluten, their immune system attacks its own tissues, leading to autoantibody production. Identifying these antibodies helps determine the likelihood of celiac disease, guiding further diagnostic steps. While effective for screening, these tests do not provide a definitive diagnosis on their own.
Types of Celiac Disease Blood Tests
The most commonly ordered blood test for celiac disease is the Tissue Transglutaminase IgA (tTG-IgA). This test measures antibodies against tissue transglutaminase, an enzyme targeted by the immune response in most individuals with celiac disease. Elevated tTG-IgA antibody levels are a strong indicator of the condition.
Another antibody test is the Endomysial Antibody (EMA-IgA). This test identifies antibodies against endomysium, the connective tissue surrounding muscle fibers. EMA-IgA is highly specific for celiac disease, but it is more expensive and labor-intensive than the tTG-IgA test. Therefore, EMA-IgA is often used to confirm positive tTG-IgA results rather than as a primary screening tool.
Deamidated Gliadin Peptide tests (DGP-IgA and DGP-IgG) measure antibodies against a modified form of gliadin, a component of gluten. These tests are useful for young children under two years, where tTG-IgA may be less accurate, or for individuals with a suspected IgA deficiency. The DGP-IgG test is especially valuable for those who cannot produce IgA antibodies.
A Total Serum IgA test is routinely performed alongside IgA-based celiac antibody tests like tTG-IgA and EMA-IgA. This test measures the overall level of IgA antibodies in the blood. Its purpose is to identify IgA deficiency, a condition where an individual produces very low or no IgA antibodies. If a person has IgA deficiency, their IgA-based celiac tests would show a false negative result, as their body cannot produce the IgA antibodies for detection.
Understanding Positive and Negative Results
A “positive” result on a celiac disease blood test indicates elevated levels of celiac-related antibodies in the bloodstream. This means the immune system is reacting to gluten, making celiac disease highly likely. The degree of elevation can sometimes correlate with the extent of intestinal damage, though this is not a definitive measure.
A “negative” result means celiac-related antibodies were not detected. This typically suggests celiac disease is unlikely, provided the individual consumed gluten regularly before the test. While reassuring, a negative result does not rule out celiac disease in all circumstances.
Sometimes, test results are reported as “borderline,” “equivocal,” or “weakly positive.” These terms indicate slightly elevated antibody levels that do not definitively fall into the positive range. Such results often suggest re-testing after continued gluten consumption, or they may prompt further diagnostic procedures. Borderline results can sometimes occur early in the disease progression or if gluten intake has been inconsistent.
Factors That Influence Test Accuracy
The accuracy of celiac disease blood tests is significantly influenced by dietary gluten intake, which is the most important factor. For reliable results, an individual must consume gluten for several weeks or months before the blood draw. This is because antibodies are produced in response to gluten exposure; without it, the immune system will not generate detectable antibodies.
Following a gluten-free or low-gluten diet before testing can lead to a false-negative result. If gluten is excluded, the small intestine may heal, and antibody levels can decrease. Therefore, it is recommended to consume gluten daily for at least 4-6 weeks before testing, equivalent to a few slices of bread per day.
False positives, though less common, can occur. Other autoimmune conditions, such as Type 1 diabetes or autoimmune thyroid disease, or certain liver diseases, can occasionally cause a slight elevation in celiac-related antibody levels without celiac disease being present. These situations highlight why a positive blood test is a screening tool, not a final diagnosis.
IgA deficiency can also lead to inaccurate test results, specifically false negatives for tTG-IgA and EMA-IgA tests. Since these tests rely on IgA antibodies, a person with an IgA deficiency will not produce them, regardless of whether they have celiac disease. This is why the Total Serum IgA test is performed concurrently, to identify individuals who might require DGP-IgG testing instead.
Next Steps in the Diagnostic Process
When a celiac disease blood test is positive, it is not a final diagnosis. The standard next step is a referral to a gastroenterologist, a specialist in digestive disorders, who will typically recommend an upper endoscopy with a small intestine biopsy. During the endoscopy, a flexible tube with a camera is passed into the small intestine. Small tissue samples, or biopsies, are taken and examined under a microscope for characteristic signs of celiac disease, such as flattened villi. The biopsy is the definitive method for confirming a celiac disease diagnosis by directly observing intestinal damage.
If blood tests are negative but symptoms persist, a doctor might explore other diagnoses with similar symptoms, such as irritable bowel syndrome or lactose intolerance. In some cases, genetic testing for HLA-DQ2 and HLA-DQ8 genes may be considered. While these genes are necessary for celiac disease to develop, their absence makes the diagnosis highly unlikely.