Celiac disease testing typically starts with a simple blood test that looks for specific antibodies your immune system produces in response to gluten. If that blood test comes back positive, the next step is usually an upper endoscopy with a small intestinal biopsy to confirm the diagnosis. The most important thing to know before getting tested: you need to be eating gluten regularly, or the tests won’t work.
The First Step: Blood Tests
The standard screening test is called the tTG-IgA, which measures antibodies your body makes when it reacts to gluten. This test has a sensitivity of 78% to 100% and a specificity of 90% to 100%, making it a strong first-line screen. Your doctor will typically order this alongside a total IgA level, because about 2% to 3% of people with celiac disease have IgA deficiency, a condition where the body doesn’t produce enough of one type of antibody. If your IgA levels are very low, the standard test can give a falsely negative result.
For people with IgA deficiency, doctors switch to IgG-based versions of the same tests. These include the DGP-IgG (which detects antibodies against a specific gluten fragment) and the tTG-IgG. These alternatives catch what the standard panel misses, though a positive IgG result still requires a biopsy to confirm the diagnosis.
A second antibody test, the EMA-IgA, is sometimes used as a follow-up to the tTG-IgA. It has a specificity of 97% to 100%, meaning a positive result is very reliable. When both tests come back positive, the likelihood of celiac disease is high.
If your tTG-IgA is negative and your IgA levels are normal, celiac disease is considered effectively ruled out for most people. The exception is anyone with a strong clinical suspicion, such as a first-degree relative with celiac disease or symptoms that strongly suggest it. In those cases, a biopsy may still be warranted even with negative blood work.
Why You Must Be Eating Gluten
This is where many people run into trouble. If you’ve already cut gluten out of your diet before testing, your antibody levels may have dropped to normal and your intestinal lining may have started healing. Both the blood tests and the biopsy depend on your body actively reacting to gluten.
Current guidelines recommend eating gluten for at least 14 days, consuming more than 3 grams per day (roughly one to two slices of bread). Some protocols use a lower dose of gluten over a longer period, around 3 months, for people who find a full gluten challenge too uncomfortable. If you’ve been gluten-free for a while and need a diagnosis, talk to your doctor about the best approach before reintroducing gluten on your own.
Confirming the Diagnosis With a Biopsy
A positive blood test leads to an upper endoscopy, where a gastroenterologist passes a thin, flexible tube through your mouth into the upper part of your small intestine (the duodenum). During this procedure, they take several tiny tissue samples. The whole thing usually takes 15 to 20 minutes, and you’re sedated for it.
The tissue samples are examined under a microscope and graded on a scale called the Marsh classification. At the low end (Marsh 1), the intestinal lining shows increased immune cells but no structural damage. At the high end (Marsh 3c), the tiny finger-like projections that line the intestine (villi) are completely flattened, which is what prevents nutrient absorption and causes many celiac symptoms. A Marsh 3 finding with positive blood work is a definitive celiac diagnosis. Marsh 1 or 2 findings can be trickier to interpret and may require additional evaluation.
The biopsy matters because a small percentage of people with positive blood tests turn out to have a different condition, and some people with negative blood tests actually do have celiac disease. Studies estimate that truly seronegative celiac disease (negative blood work but positive biopsy) accounts for roughly 2% to 3% of all celiac cases, though some older estimates put the number higher.
Genetic Testing: Ruling It Out
Genetic testing for celiac disease works differently from the blood test and biopsy. It doesn’t tell you whether you have the disease. Instead, it tells you whether you could develop it. Celiac disease requires one of two genetic markers: HLA-DQ2 or HLA-DQ8. About 99.5% of people with celiac disease carry one or both of these genes.
The real power of this test is its ability to rule celiac disease out. If you don’t carry either gene, your chance of developing celiac disease is extremely low. This makes genetic testing especially useful for family members of someone with celiac disease. About 40% of the general population carries DQ2 or DQ8, so a positive genetic test doesn’t mean much on its own. But a negative result can spare first-degree relatives from years of repeated antibody screening. It can also eliminate about 60% of at-risk individuals from needing ongoing monitoring.
Testing Children
Children follow a similar diagnostic path, but with one notable difference. European guidelines from ESPGHAN allow a no-biopsy diagnosis in children when the tTG-IgA level is at least 10 times the upper limit of normal and a second blood sample confirms a positive EMA-IgA result. When both criteria are met, the diagnosis is considered reliable enough to skip the endoscopy.
This no-biopsy approach only applies when the antibody levels are very high and confirmed on a separate blood draw. Children with elevated but lower antibody levels still need a biopsy. In the United States, many pediatric gastroenterologists have adopted this approach, though practices vary. The threshold of 10 times the upper limit must be measured with specific lab methods that can accurately quantify antibody levels at that range.
Diagnosis Through the Skin
Some people with celiac disease develop a distinctive, intensely itchy rash called dermatitis herpetiformis, which typically appears on the elbows, knees, buttocks, and scalp. This rash can be diagnosed with a skin biopsy, but the technique is specific: the sample must be taken from normal-appearing skin right next to a lesion, not from the rash itself.
Under a special staining technique called direct immunofluorescence, the biopsy reveals granular IgA deposits in the skin. This finding is positive in about 92% of patients with dermatitis herpetiformis and is considered the gold standard for diagnosing the condition. Because dermatitis herpetiformis is a direct manifestation of celiac disease, a confirmed skin biopsy is enough to diagnose celiac disease without a separate intestinal biopsy.
What Can Make Testing Unreliable
Several factors can produce misleading results. The most common is going gluten-free before testing, which can normalize both blood work and biopsy findings. IgA deficiency, as mentioned earlier, will cause false negatives on the standard tTG-IgA test if the doctor doesn’t check total IgA levels at the same time.
Age also plays a role. Children under two may not yet produce reliable levels of tTG-IgA antibodies, so testing in very young children sometimes includes DGP-IgG as an additional marker. Certain medications that suppress the immune system can also lower antibody levels, potentially masking the disease.
If you’ve already been eating gluten-free and your doctor wants to test you, the gluten challenge can feel daunting. For people who react severely to gluten, even shorter or lower-dose challenges (as little as 60 to 120 mg per day for three months) have shown promise in triggering a diagnosable immune response, though the standard recommendation remains at least 3 grams daily for two weeks.