Celiac disease is diagnosed through a combination of blood tests and, in most cases, a small intestine biopsy. The process starts with a simple blood draw that measures specific antibodies your body produces in response to gluten. If those results come back elevated, an upper endoscopy with tissue samples typically follows to confirm the diagnosis.
The First Step: Antibody Blood Tests
The standard screening test measures an antibody called tissue transglutaminase IgA (tTG-IgA). When someone with celiac disease eats gluten, their immune system produces this antibody as it attacks the lining of the small intestine. The test catches about 93% of people who have celiac disease and correctly rules it out about 97% of the time. In children, accuracy is even higher, with sensitivity around 96%.
Your doctor should also check your total IgA level at the same time. About 2 to 3% of people with celiac disease have IgA deficiency, a condition where the body doesn’t produce enough of this particular type of antibody. If that’s the case, the standard tTG-IgA test can come back falsely normal. For people with IgA deficiency, doctors use IgG-based versions of the test instead.
One critical rule: you must be eating gluten regularly when you get tested. If you’ve already cut gluten from your diet, antibody levels drop and the blood test may miss the disease entirely.
Confirming the Diagnosis With a Biopsy
A positive blood test alone isn’t usually enough for a definitive diagnosis in adults. The next step is an upper endoscopy, where a gastroenterologist passes a thin, flexible tube through your mouth and into your small intestine. The procedure typically takes 15 to 20 minutes, and you’re sedated for it.
During the endoscopy, the doctor collects multiple small tissue samples from your duodenum, the first section of the small intestine. Current guidelines call for one or two samples from the bulb (the very beginning) and at least four from farther down. Taking this many samples matters because celiac damage can be patchy, affecting some areas and sparing others.
A pathologist then examines the tissue under a microscope and assigns a Modified Marsh score ranging from 0 to 3c. The score reflects how much damage gluten has caused to the tiny finger-like projections (villi) that line your intestine and absorb nutrients. A score of 3 or above, where those projections are significantly flattened, is the threshold most doctors use to confirm celiac disease.
When Children Can Skip the Biopsy
Children have an alternative path to diagnosis. Guidelines from the European Society for Paediatric Gastroenterology allow a no-biopsy diagnosis when the tTG-IgA level is extremely high, at least 10 times the upper limit of normal. A second blood sample must also test positive for a different antibody called endomysial antibody (EMA) to confirm the result. The American College of Gastroenterology has adopted similar criteria for children and notes that symptomatic adults who are unwilling or unable to undergo endoscopy may also be considered for this approach.
This no-biopsy pathway exists because antibody levels that high are almost always associated with significant intestinal damage. It spares young children from sedation and an invasive procedure when the blood work is already overwhelming.
What Genetic Testing Can (and Can’t) Tell You
Celiac disease requires specific genetic markers called HLA-DQ2 and HLA-DQ8. About 30 to 40% of the general population carries one of these genes, so having them doesn’t mean you have celiac disease. But not having them essentially rules it out. That’s what makes genetic testing useful: its negative predictive value is extremely high.
Genetic testing is most helpful in a few specific situations. If you have a first-degree relative with celiac disease, a negative gene test means you don’t need ongoing monitoring. It’s also useful when blood tests and biopsy results are inconclusive, or when someone has already started a gluten-free diet before being tested, making antibody and biopsy results unreliable. Unlike antibody tests, genetic results aren’t affected by what you’re eating.
What to Do If You’ve Already Gone Gluten-Free
This is one of the most common testing complications. If you stopped eating gluten before getting tested, your antibody levels may have already returned to normal and your intestinal lining may have started healing. Both the blood test and biopsy can come back negative even if you do have celiac disease.
The solution is a gluten challenge: deliberately eating gluten for a set period before testing. The traditional approach involves consuming roughly 8 to 10 grams of gluten per day (about four to six slices of bread) for six to eight weeks. A modified version uses a smaller dose of about 3 grams per day (roughly one and a half slices of bread), which is better tolerated and still produces reliable results.
If you develop symptoms quickly, your doctor may recheck antibody levels two to four weeks in. If those come back negative, you continue the challenge and retest at the eight-week mark, or even up to 12 weeks from the start. The process can be uncomfortable for people who are genuinely gluten-sensitive, but it’s currently the only way to get accurate results after gluten has been removed from the diet.
Testing for IgA Deficiency
Because IgA deficiency is more common in people with celiac disease than in the general population, checking total IgA alongside the tTG-IgA test is standard practice. If your IgA levels are low, the regular celiac screening antibodies won’t work properly.
In that case, doctors switch to IgG-based tests. The two most commonly used are the tTG-IgG and the deamidated gliadin peptide IgG (DGP-IgG). These measure different antibodies that aren’t affected by IgA deficiency. If either comes back positive, the same biopsy pathway follows.
What the Full Diagnostic Path Looks Like
For most people, the process follows a clear sequence. You get a blood draw for tTG-IgA and total IgA while eating a normal, gluten-containing diet. If tTG-IgA is elevated and IgA levels are normal, you’re referred for an upper endoscopy with duodenal biopsies. If the biopsy shows villous damage at Marsh 3 or above, you have a confirmed celiac diagnosis.
If blood tests are positive but the biopsy looks normal, your doctor may recommend genetic testing, repeat testing over time, or evaluation for other conditions that can elevate celiac antibodies. If blood tests are negative but clinical suspicion is strong, genetic testing can help determine whether celiac disease is even possible. A negative result for both HLA-DQ2 and HLA-DQ8 makes the diagnosis extremely unlikely and usually ends the workup.
The entire process from initial blood draw to confirmed diagnosis typically takes a few weeks, though a gluten challenge can extend that timeline to three months or more. Once diagnosed, most people start a strict gluten-free diet and have follow-up antibody testing after six to twelve months to make sure levels are dropping.