tTG-IgA test: the first-line celiac disease blood test, what the levels mean, and what happens after a positive result

In celiac disease, the immune system reacts to gluten by producing antibodies against tissue transglutaminase, an enzyme in the lining of the small intestine that modifies gluten fragments and makes them more immunogenic. The tTG-IgA test measures the immunoglobulin A antibodies directed at that enzyme, reported in units per milliliter (U/mL) against a laboratory cutoff — commonly around 4 U/mL for a negative result, with weak-positive and positive bands above it.

The test is favored because it combines high sensitivity (about 90–98% in people eating gluten) with high specificity (about 95–99%), it is inexpensive and automated, and it falls as the intestine heals on a gluten-free diet, so it doubles as a monitoring test. Related celiac antibodies — deamidated gliadin peptide (DGP) IgA and IgG, endomysial antibody (EMA) and tTG-IgG — are used to confirm a result or to cover the situations where tTG-IgA fails.

It is an antibody test, not a genetic test. HLA-DQ2/DQ8 typing answers a different question — whether celiac disease is possible at all (about 40% of the population carries the genes; nearly all people with celiac disease do) — and is used mainly to rule celiac out in ambiguous cases or in people already on a gluten-free diet.

The American College of Gastroenterology recommends testing people with symptoms or findings that celiac disease can cause, and people in high-risk groups even without symptoms. The classic presentation — diarrhea, bloating, weight loss and malabsorption — is now the minority. Most adults diagnosed today present with iron-deficiency anemia that does not respond to iron, unexplained fatigue, osteoporosis at a young age, persistent mouth ulcers, elevated liver enzymes, infertility or recurrent miscarriage, peripheral neuropathy, migraine, or the itchy blistering rash dermatitis herpetiformis.

High-risk groups in whom testing is recommended regardless of symptoms include first-degree relatives of someone with celiac disease (about 1 in 10 are affected), people with type 1 diabetes, autoimmune thyroid disease, Down, Turner or Williams syndrome, selective IgA deficiency, and unexplained low bone density.

People who feel unwell after eating gluten but have not been tested are a common reason for ordering the test directly — and the right sequence matters. Testing must happen before gluten is removed from the diet, because the antibodies disappear on a gluten-free diet and a negative result then proves nothing.

tTG-IgA antibodies are produced in response to ongoing gluten exposure. When gluten is removed, antibody levels fall — often below the cutoff within months — even though the disease is still present. Someone who has been gluten-free for several weeks can therefore test falsely negative.

Guidelines advise staying on a gluten-containing diet until all testing, including any biopsy, is complete. For someone already gluten-free who wants a diagnosis, the options are a supervised gluten challenge (commonly one to two slices of wheat bread daily for at least two to six weeks before re-testing, with duration set by the clinician) or HLA-DQ2/DQ8 genetic testing, which can rule celiac disease out if negative but cannot confirm it if positive.

Some drugs also blunt the response: immunosuppressants such as prednisone, azathioprine or biologic therapies can lower antibody levels and should be noted on the request.

About 2–3% of people with celiac disease have selective IgA deficiency — ten to fifteen times the rate in the general population. Because they cannot make IgA-class antibodies, their tTG-IgA is falsely negative no matter how active the disease. Every celiac work-up therefore includes a total serum IgA, and if it is low the laboratory switches to IgG-based tests: tTG-IgG and DGP-IgG.

Most celiac screening panels, including TestWell's, bundle total IgA with tTG-IgA for this reason. A tTG-IgA ordered alone with no IgA measurement leaves the most common cause of a false negative unchecked.

Laboratories report a value with an interpretation band. The exact numbers vary by assay, so the bands on your report govern, but the pattern is consistent:

For adults, US guidelines still recommend confirming a positive antibody test with an upper endoscopy and duodenal biopsies while gluten remains in the diet. The biopsy establishes the baseline damage, excludes look-alike conditions, and matters because the treatment — strict lifelong gluten avoidance — is demanding enough that the diagnosis should be certain. In children, the European pediatric society's no-biopsy pathway allows diagnosis when tTG-IgA is at least ten times the upper limit and a separate EMA test is positive; some US centers apply similar criteria selectively.

Once treatment starts, tTG-IgA is used to follow it. Levels typically fall substantially within six to twelve months on a strict gluten-free diet and normalize in most people within one to two years; a level that stays high usually means ongoing gluten exposure, often unintentional. A repeat test at six to twelve months and then annually is a common schedule. Newly diagnosed people are also checked for the deficiencies celiac disease causes — iron, ferritin, vitamin B12, folate, vitamin D — and for bone density when indicated.

tTG-IgA is a routine venous blood draw with no fasting, available at Quest and Labcorp with results in a few business days. It can be ordered by a clinician or directly through TestWell, where the celiac screening panel pairs tTG-IgA with total IgA (and, in the comprehensive version, the DGP and tTG-IgG antibodies that cover IgA deficiency). Keep eating gluten until the result is back and any biopsy is done, and bring the report to a gastroenterologist if it is positive.