Tissue Transglutaminase IgA (tTG-IgA)
This is the main blood test used when coeliac disease is being investigated, an immune reaction to gluten. It looks for an antibody your body makes when gluten is harming the lining of your small bowel. This only works if you are still eating gluten when you give blood. If you have already cut gluten out, it is worth pausing and talking to your GP before testing, because going gluten-free can make the result come back falsely clear even if you do have coeliac disease. A positive result is a prompt for the next step, not a diagnosis on its own.
Standard CareWhat is tissue transglutaminase iga (ttg-iga)?
Tissue transglutaminase IgA (tTG-IgA) is a serology test — a blood test that looks for a specific antibody (a protein made by the immune system). In coeliac disease (a genetic autoimmune condition where the immune system reacts to gluten, a protein found in wheat, rye, and barley), the gut lining is damaged as part of this immune attack. One of the targets is an enzyme called tissue transglutaminase (tTG), which is found throughout the gut wall. The immune system produces IgA antibodies against it, and the tTG-IgA test measures the level of those antibodies in the blood. The test is the first-line screening marker recommended for coeliac disease in Australian clinical practice. It is ordered alongside a total IgA level — a separate measurement of your overall IgA immunoglobulin — because roughly 2–3% of people with coeliac disease have selective IgA deficiency (a condition where the body does not produce enough IgA of any kind). If total IgA is low, the tTG-IgA test can return a falsely negative result even when coeliac disease is present, and your GP will use a different type of antibody test instead. The name includes "tissue" because the target is an enzyme in the gut wall, not because the test itself involves a tissue sample. It is a straightforward blood draw.
Why is it tested?
A tTG-IgA test is ordered when coeliac disease is being investigated. Common reasons include unexplained or persistent gut symptoms (such as bloating, cramping, diarrhoea, or nausea), unexplained anaemia, unintentional weight loss, fatigue, or poor growth in children. It may also be ordered in someone with a first-degree family member (parent, sibling, or child) who has already been diagnosed with coeliac disease, since the condition runs in families. The test is also used in people already diagnosed with coeliac disease to monitor how well a gluten-free diet is being followed. When someone sticks to a strict gluten-free diet, tTG-IgA levels usually fall over time; persistent elevation suggests ongoing gluten exposure. One essential condition for an accurate result: it is only reliable if you have been eating a normal, gluten-containing diet in the weeks before the blood test. Your GP will usually advise you to keep eating gluten until your coeliac testing is finished, because going gluten-free first lowers the antibody level and can produce a falsely negative result. If you have already started a gluten-free diet and want to confirm or rule out coeliac disease, raise it with your GP before testing rather than relying on the result.
Reference range (Australia)
The cut-off for a positive result varies by laboratory and assay — there is no single national or international standard. The reference interval printed on your own laboratory report is the correct one to use for your result. Do not compare your number to a cut-off from a different lab or a general website.
Reference intervals vary between laboratories. Always use the range printed on your own report.
What your result means
A negative (low or undetectable) tTG-IgA result, when your total IgA is normal, generally makes active coeliac disease less likely. Australian guidelines note that coeliac disease serology has a false negative rate of around 10–15%, so a negative result does not completely rule out the condition — particularly if you have strong symptoms or a family history. Two situations can produce a falsely negative result: First, if you have already started a gluten-free diet, the antibody levels can fall rapidly, and the test may not detect a problem that was previously present. Testing is most meaningful on a normal, gluten-containing diet. Second, if your total IgA level is low (selective IgA deficiency), the tTG-IgA test may return a negative result even if coeliac disease is present. For this reason, total IgA is always tested at the same time. If total IgA is low, your GP can order IgG-based coeliac antibody tests instead. If clinical suspicion remains high despite a negative result, your GP may explore further options.
A normal tTG-IgA result, interpreted alongside a normal total IgA, is generally reassuring — it makes active coeliac disease less likely. Keep in mind that the result is only reliable if you were eating gluten at the time of the test. If your symptoms persist, it is worth a conversation with your GP, who will consider the full picture including your symptoms, diet, and family history.
A positive (elevated) tTG-IgA result means the immune system is producing antibodies against tissue transglutaminase — a pattern strongly associated with coeliac disease in someone who has been eating gluten. The higher the level, the greater the concern, though the level alone does not confirm a diagnosis. A positive antibody result alone is not enough to diagnose coeliac disease. Australian clinical guidelines recommend referral to a gastroenterologist (a gut specialist) for a confirmatory small bowel biopsy (gastroscopy) — a procedure in which small tissue samples are taken from the duodenum (the first part of the small intestine) and examined under a microscope for the characteristic gut-lining damage called villous atrophy. Positive serology combined with biopsy findings confirms the diagnosis. Your GP will also check that your total IgA is normal, because a low total IgA can make the tTG-IgA read falsely negative even when coeliac disease is present. They may also consider other causes of a mildly elevated tTG-IgA (such as other autoimmune conditions or gut inflammation). Your GP can tell you what the result means for your specific situation.
What can affect your result
The most important factor is your gluten intake. The tTG-IgA test reflects an active immune response to gluten. Antibody levels rise when gluten is being consumed and the gut immune response is active; they fall when gluten is removed from the diet. For this reason, the test is only reliable when it is done while you are eating a normal gluten-containing diet, which is why your GP advises continuing gluten until coeliac testing is complete. Starting a gluten-free diet before testing is the most common reason for a falsely negative result. Total IgA is the second critical variable. If your total IgA level is low (a condition called selective IgA deficiency, affecting around 2–3% of people with coeliac disease), your immune system does not produce enough IgA of any type, and tTG-IgA may be undetectable even if coeliac disease is present. Your GP will always measure total IgA alongside tTG-IgA to catch this scenario. Other autoimmune conditions — such as type 1 diabetes, thyroid disease, or inflammatory bowel disease — can occasionally produce mildly elevated tTG-IgA in people who do not have coeliac disease. This is one reason why a positive result requires biopsy confirmation rather than diagnosis by serology alone. In people already diagnosed with coeliac disease and following a gluten-free diet, tTG-IgA levels typically normalise within 6–12 months. Levels that remain persistently elevated generally suggest ongoing gluten exposure, and your GP or dietitian can help identify hidden sources.
When to act
If your tTG-IgA is elevated, your GP will review your total IgA level and consider a referral to a gastroenterologist for confirmatory small bowel biopsy — the step that confirms or rules out coeliac disease. One thing worth knowing: going gluten-free before the biopsy is complete can make the gut changes harder to detect, so your GP will guide the timing. If your total IgA came back low, your GP may order IgG-based coeliac antibody tests and discuss next steps with you. A result where both tTG-IgA and total IgA are low together needs specialist interpretation. If your tTG-IgA is negative but you still have symptoms that concern you, raise this at your next visit. Your GP will weigh up whether you were eating gluten regularly before the test, whether your total IgA was normal, and whether further investigation is warranted. If you have already been diagnosed with coeliac disease, a rising tTG-IgA on a gluten-free diet is worth discussing with your GP or dietitian — it often indicates a hidden source of gluten in your diet. This page is general information and is not a substitute for advice from your GP.
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