Celiac disease is diagnosed through a combination of blood tests and, in most adult cases, a small intestinal biopsy. The standard first step is a blood test called the tTG-IgA, which detects antibodies your immune system produces in response to gluten. This test catches about 93% of celiac cases and correctly rules it out about 98% of the time. What matters most before any testing begins: you need to be eating gluten regularly, or the results will be unreliable.
The tTG-IgA Blood Test
The tissue transglutaminase IgA test is the go-to screening tool for celiac disease. It measures antibodies that your body makes when your immune system reacts to gluten. A positive result doesn’t confirm celiac on its own, but it’s a strong signal that further evaluation is needed.
Most labs will also check your total IgA level at the same time. About 2 to 3% of people with celiac disease have a condition called selective IgA deficiency, meaning their body doesn’t produce enough IgA antibodies to register on the standard test. If your total IgA is very low or undetectable, the tTG-IgA will come back falsely negative. In that case, your doctor will order IgG-based alternatives: either a tTG-IgG test or a deamidated gliadin peptide (DGP) IgG test, both of which bypass the IgA issue entirely.
A second antibody test, called the EMA (endomysial antibody) test, is sometimes used alongside or after the tTG-IgA. When combined, these two tests approach nearly 100% sensitivity and specificity. The EMA test plays a particularly important role in confirming the diagnosis in children who may qualify for a biopsy-free diagnosis.
You Must Be Eating Gluten Before Testing
This is the single biggest mistake people make. All celiac blood tests require you to be actively eating gluten for the antibodies to show up. If you’ve already cut gluten from your diet, your antibody levels may have dropped enough to produce a false negative. The North American Society for Pediatric Gastroenterology recommends eating roughly two slices of wheat-based bread per day for six to eight weeks before blood testing. For biopsy, the University of Chicago’s Celiac Disease Center recommends at least one slice of bread daily for a minimum of two to three weeks.
If you’ve already gone gluten-free and want an accurate diagnosis, you’ll need to do what’s called a gluten challenge: deliberately reintroduce gluten before testing. This is where things get difficult. Many people refuse because eating gluten again triggers the exact symptoms they eliminated. Current guidelines suggest anywhere from 3 grams per day for two to eight weeks to 10 grams per day for six to eight weeks, but there’s no universally standardized protocol. The longer the challenge, the more reliable the biopsy results, but the harder it is to sustain.
Intestinal Biopsy
For most adults with a positive blood test, the next step is an upper endoscopy with biopsies of the small intestine. During the procedure, a thin flexible tube is passed through your mouth into the upper part of your small intestine (the duodenum). You’re sedated for this, and the whole thing typically takes 15 to 20 minutes.
At least six tissue samples should be taken from multiple areas of the duodenum. This matters because celiac damage can be patchy, so sampling from only one spot risks missing it. The pathologist examines the samples for a progression of changes: increased immune cells in the intestinal lining, elongation of the tissue’s deeper structures, and eventually flattening of the tiny finger-like projections (villi) that line the intestine and absorb nutrients. These changes are graded on something called the Marsh classification, from mild immune cell infiltration (Marsh I) through partial or total villous flattening (Marsh III). A Marsh III result with positive blood work is considered definitive.
Diagnosis in Children Without Biopsy
Children may not need an endoscopy at all. Guidelines from both the European Society for Paediatric Gastroenterology (ESPGHAN) and the 2023 American College of Gastroenterology now support a biopsy-free diagnosis in pediatric patients when two conditions are met: the tTG-IgA level is more than 10 times the upper limit of normal, and a second blood sample confirms a positive EMA test. When antibody levels are that high, the likelihood of celiac disease is strong enough that putting a child through sedation and endoscopy isn’t necessary.
One note for very young children: kids need to have been eating wheat or barley-based foods for some time, potentially up to a year, before their immune system generates enough of an autoimmune response to show up on testing. Testing too early in a toddler who has only recently started eating solids can produce misleading results.
Genetic Testing: Ruling It Out
Genetic testing for celiac disease works differently from the other tests. It doesn’t tell you whether you have celiac disease. It tells you whether you could ever develop it. Celiac disease requires one of two genetic markers: HLA-DQ2 or HLA-DQ8. If you don’t carry either gene, you can essentially rule out celiac disease with near-100% certainty, regardless of what you’re eating at the time of the test.
The catch is that carrying one of these genes doesn’t mean much on its own. Roughly 40% of the general population has HLA-DQ2 or HLA-DQ8, and the vast majority of them will never develop celiac disease. So a positive genetic test has very low predictive value. Its real usefulness is in specific situations: screening family members of someone with celiac, or resolving uncertain cases where blood work and biopsy results conflict. A negative result can spare you from further testing, including endoscopy and gluten challenges.
Testing if You’re Already Gluten-Free
This is one of the most frustrating scenarios in celiac diagnosis. Many people start feeling better after removing gluten and only then wonder whether they actually have celiac disease. At that point, standard blood tests and biopsies may no longer show the characteristic damage.
Genetic testing is the one option that doesn’t require you to be eating gluten. If you test negative for HLA-DQ2 and HLA-DQ8, celiac disease is effectively ruled out and you can stop there. If you do carry the genes, a formal gluten challenge followed by blood testing and biopsy is the standard path to diagnosis. The challenge typically involves eating the equivalent of several slices of bread daily for at least six to eight weeks, though some newer research is exploring shorter, higher-dose protocols. The experience is often unpleasant for people who truly have celiac disease, and a significant number of patients decline or quit the challenge early because symptoms return.
Getting a confirmed diagnosis matters for more than curiosity. It affects how aggressively you need to avoid trace gluten, whether your family members should be screened, and whether you’ll receive monitoring for the nutritional deficiencies and associated conditions that come with celiac disease long-term.
What About Home Test Kits?
Home finger-prick test kits for celiac disease are available over the counter and online. Most work by detecting tTG-IgA antibodies in a small blood sample, similar in principle to the lab test. However, their sensitivity is generally lower than laboratory-based testing, and a negative result on a home kit does not reliably rule out celiac disease. A positive home test still needs to be followed up with standard lab work and a clinical evaluation. These kits can serve as a low-barrier first step if you’re hesitant to see a doctor, but they aren’t a substitute for the full diagnostic process.