Celiac disease testing typically starts with a simple blood test and, if results are positive, is confirmed with a biopsy taken during an upper endoscopy. The most common screening blood test, called tTG-IgA, has a sensitivity between 78% and 100%, making it a reliable first step. Getting accurate results depends on one crucial detail: you need to be eating gluten regularly before and during the testing process.
The Blood Test That Starts the Process
The first test your doctor will order is a blood draw checking for specific antibodies your immune system produces when it reacts to gluten. The primary test measures an antibody called tissue transglutaminase IgA (tTG-IgA), which has a specificity of 90% to 100%. That means a positive result very rarely turns out to be a false alarm.
There’s one catch. About 2% to 3% of people with celiac disease have a condition called selective IgA deficiency, meaning their bodies don’t produce enough of the IgA antibody class for the standard test to detect. If your total IgA level comes back low, your doctor will order an alternative test that looks for IgG-based antibodies instead. This is why many labs automatically check your total IgA level alongside the tTG-IgA.
A negative blood test is generally reassuring, but it isn’t perfect. If your doctor still suspects celiac based on your symptoms or family history, they may move forward with additional testing anyway.
Why You Must Keep Eating Gluten
This is the part that trips people up. If you’ve already cut gluten from your diet before getting tested, your antibody levels may have dropped enough to produce a false negative. The blood test and biopsy both look for your body’s active response to gluten, so they need gluten in your system to work.
If you’ve gone gluten-free and need to get tested, you’ll be asked to do what’s called a gluten challenge. The traditional approach involves eating the equivalent of about four to six slices of bread per day (8 to 10 grams of gluten) for six to eight weeks before testing. A modified version, tested in adults with biopsy-proven celiac disease, reduces that to about 1.5 slices of bread per day (3 grams of gluten). You can gradually build up your intake over the first week rather than jumping to the full amount immediately.
For people with symptoms, a doctor may recheck antibodies as early as two to four weeks into the challenge. If that result is positive, you can likely stop the challenge and proceed to a biopsy. If it’s negative, you’ll typically continue eating gluten and retest around 12 weeks from when you started.
Confirming the Diagnosis With a Biopsy
A positive blood test alone isn’t enough for an official diagnosis in most adults. The next step is an upper endoscopy, a procedure where a thin, flexible tube with a camera is passed through your mouth and into your small intestine. You’ll be sedated for this, and the procedure itself takes about 15 to 20 minutes.
During the endoscopy, the doctor takes small tissue samples from your duodenum (the first section of the small intestine). Current guidelines from the British Society of Gastroenterology recommend taking at least four biopsies, including one from the duodenal bulb, which is the very beginning of the duodenum right where the stomach empties into it. Multiple samples are important because celiac damage can be patchy, appearing in some areas but not others.
A pathologist then examines these samples under a microscope, looking for characteristic damage: flattened villi (the tiny finger-like projections that absorb nutrients), increased immune cells in the intestinal lining, and other signs of inflammation. The degree of damage is graded on a scale, which helps confirm the diagnosis and gives your doctor a baseline for measuring improvement later.
When Children Can Skip the Biopsy
Pediatric guidelines from the European Society for Paediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) allow children to be diagnosed without a biopsy under specific conditions. If a child’s tTG-IgA level is at least 10 times the upper limit of normal, and a second blood sample confirms the result with a different antibody test (endomysial antibodies, or EMA), the diagnosis is considered secure without endoscopy.
This no-biopsy approach applies to both symptomatic and asymptomatic children. Genetic testing and the presence of symptoms are not required for this pathway. The key safeguard is the extremely high antibody threshold: levels that high are almost never seen in people who don’t have celiac disease. The antibody tests used must be high-quality, calibrated assays where the 10 times threshold falls within their measurement range.
Genetic Testing and Its Role
Celiac disease requires the presence of one or both of two specific gene variants: HLA-DQ2 and HLA-DQ8. Genetic testing for these markers has a high negative predictive value, meaning if you don’t carry either gene, you can essentially rule out celiac disease for life. No follow-up testing would be needed.
However, carrying one of these genes doesn’t mean you have or will develop celiac. Roughly 30% to 40% of the general population carries HLA-DQ2 or DQ8, while only about 1% develops celiac disease. That’s why genetic testing is most useful as a rule-out tool rather than a diagnostic one. It’s particularly helpful for people who started a gluten-free diet before being tested (avoiding the need for a gluten challenge if genes are negative), for family members wondering about their risk, and for patients with unclear biopsy or blood test results.
Tracking Recovery After Diagnosis
Once you’re diagnosed and start a strict gluten-free diet, your doctor will use the same tTG-IgA blood test to monitor whether your immune response is calming down. Antibody levels should normalize within about 12 months, though the timeline varies from person to person. If antibodies are still elevated at 18 months to two years, it typically signals that gluten is still getting into your diet somewhere, whether from cross-contamination, mislabeled foods, or hidden sources you haven’t identified.
Some doctors will also recommend a follow-up biopsy one to two years after diagnosis to confirm that the intestinal lining is healing. This isn’t universal practice, but it provides the most direct evidence that the damage is resolving. Beyond confirming healing, ongoing monitoring helps catch nutritional deficiencies that may linger even after you’ve removed gluten, since the intestine can take time to fully recover its ability to absorb nutrients like iron, calcium, and certain vitamins.