It is a common story. Digestive symptoms that no one could explain. A decision to cut out gluten. Some relief, or a lot. And then a reasonable question: do I have celiac disease?
It is a good question, and the answer matters. It is also much harder to answer once gluten is already gone. This article explains why, and what can still be done.
What celiac disease is
In celiac disease, eating gluten triggers a reaction that damages the small intestine. Gluten is a protein found naturally in wheat, barley, rye, and triticale, and in the many foods made from them. The treatment is a gluten-free diet, followed for life, which can relieve symptoms and allow the intestine to heal.
Because the diet is lifelong and demanding, a confirmed diagnosis is worth having. It also matters for family members and for related conditions, discussed below.
How testing works
Testing for celiac disease looks for the body's reaction to gluten. The usual first step is a blood test called tissue transglutaminase IgA, or tTG-IgA. A total IgA level is checked at the same time, because some people do not make enough IgA for the test to read correctly. IgA deficiency affects about 1 in 400 people, and a different version of the test is used for them.
If the blood test suggests celiac disease, or if suspicion remains high despite a negative result, the next step is usually an upper endoscopy with biopsies of the small intestine. Several samples are taken, because the damage can be patchy.
Why gluten has to be in the diet
Here is the key point. These tests detect the reaction to gluten, so they only work while gluten is being eaten.
In people who are eating gluten, the tTG-IgA test is highly specific, at 96 to 100 percent, with a sensitivity of 63 to 93 percent, according to guidance from the American College of Gastroenterology. Remove gluten and the reaction begins to fade. The intestine begins to heal. Both the blood test and the biopsy can then look normal in someone who does have celiac disease.
The National Institute of Diabetes and Digestive and Kidney Diseases puts it plainly: if you avoid gluten before you have testing, the test results may not be accurate. Its advice is to talk with your doctor about testing first, rather than starting a gluten-free diet on your own.
If you are already gluten-free
This is not a dead end. There is a useful next step.
A genetic test can narrow the question. Celiac disease almost always occurs in people who carry certain genes, called HLA-DQ2 and HLA-DQ8. This test does not depend on eating gluten. If you do not carry either gene, current or future celiac disease is very unlikely, and the question is largely answered.
A positive genetic result does not diagnose celiac disease. These genes are common, and most people who carry them never develop the condition. A positive result simply means celiac disease remains possible.
A gluten challenge may be the next step. For someone who carries the genes and wants a definite answer, a clinician may suggest reintroducing gluten for a period before testing. The amount and the length of time should be set and supervised by your clinician, since symptoms can return. The genetic test helps identify who would not benefit from going through this.
Why the answer matters beyond you
A confirmed diagnosis has effects beyond your own diet. Testing can be considered for first-degree relatives of someone with celiac disease, even without symptoms. Testing is also worth considering in people with unexplained iron deficiency, bone loss, raised liver enzymes, or thyroid disease, because celiac disease can be behind each of them. Iron deficiency has other causes that matter, including slow blood loss from the digestive tract, so a celiac test is one part of evaluating it, not the whole evaluation. The two are linked in the other direction too: having celiac disease raises the likelihood of Hashimoto's disease, the most common cause of an underactive thyroid.
What to do now
If you have digestive symptoms and have not changed your diet, ask about celiac testing before you remove gluten. If you have already gone gluten-free, ask whether genetic testing makes sense as a first step. Either way, a registered dietitian can help you follow a gluten-free diet safely once you have a clear answer.
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This article is general education. It is not medical advice and does not replace care from your own clinician.
Sources
National Institute of Diabetes and Digestive and Kidney Diseases. Eating, Diet, & Nutrition for Celiac Disease. Last reviewed October 2020.
Arnold MJ. Diagnosis and Management of Celiac Disease: Guidelines From the American College of Gastroenterology. American Family Physician. 2024;109(1):92–93.
National Institute of Diabetes and Digestive and Kidney Diseases. Testing for Celiac Disease. Provider sheet, NIH Publication No. 13-7351, September 2013.
National Institute of Diabetes and Digestive and Kidney Diseases. Hashimoto's Disease. Last reviewed June 2021.
Ko CW, Siddique SM, Patel A, et al. AGA Clinical Practice Guidelines on the Gastrointestinal Evaluation of Iron Deficiency Anemia. Gastroenterology. 2020.
