How Celiac Disease Is Diagnosed Without Guesswork

Celiac disease is diagnosed by measuring a specific blood antibody while the person is still eating gluten, then, in most adults, confirming that result with a biopsy of the small intestine. The 2023 American College of Gastroenterology (ACG) guideline names one starting test: tissue transglutaminase IgA, known as tTG-IgA, drawn together with a total IgA level. The most common way the whole process fails is testing someone who has already removed gluten, which can turn a genuine case of celiac disease into a normal-looking result. Ordered correctly, the workup leaves little room for guesswork.

Why tTG-IgA and total IgA are ordered together

The tTG-IgA test is sensitive and specific enough that the ACG guideline places it first for almost anyone with symptoms or risk factors that raise suspicion for celiac disease. It measures an antibody of the IgA class, and that detail is exactly why a second value has to travel with it. The tTG-IgA can only rise if the body produces IgA in the first place. Selective IgA deficiency is more common among people with celiac disease than in the general population, so a person who makes little IgA can show a low tTG-IgA whether or not the disease is present. When the total IgA comes back low, the guideline shifts to IgG-based tests, such as deamidated gliadin peptide IgG or tTG-IgG, which do not depend on the antibody class that is missing. Drawing both numbers at once is what keeps a false negative from slipping through on the first blood draw.

The gluten-containing diet is the load-bearing condition

Every celiac antibody test and every biopsy finding depends on active gluten exposure. The antibodies the tests detect and the intestinal damage the biopsy grades are both driven by the immune reaction to gluten. Remove gluten and the antibodies decline over the following months while the intestinal lining begins to recover, which is why a gluten-free diet can blunt or erase the very findings the tests are meant to catch. The NIH's National Institute of Diabetes and Digestive and Kidney Diseases states plainly that a gluten-free diet should not be started before testing because it can affect the results.

This is where well-meaning trials of a gluten-free diet cause the most trouble. Someone who feels better off gluten and then asks to be tested may have normal antibodies and a normal biopsy despite truly having the disease. For a person who has already stopped, the guideline describes a gluten challenge before retesting: reintroducing daily gluten, roughly the equivalent of one to two slices of wheat bread, over a period of weeks so the immune signal has time to return. The exact duration is individualized, and a challenge in anyone with a strong prior reaction is worth doing under medical supervision.

When the guideline allows a diagnosis without a biopsy

For children, the ACG guideline supports a no-biopsy diagnosis when the tTG-IgA is very high, at or above ten times the upper limit of normal, and a separate blood sample is positive for endomysial antibody (EMA). The second sample and the second antibody guard against a labeling or laboratory error on a diagnosis that commits a child to a lifelong diet.

For adults, biopsy remains the standard. The guideline does extend a conditional option: in an adult who is unwilling or unable to undergo endoscopy, the same combination of tTG-IgA at or above ten times the upper limit of normal plus a positive EMA on a second sample supports a diagnosis of likely celiac disease. Two cautions keep this from becoming guesswork. The ten-times threshold is tied to a particular laboratory's upper limit of normal, so the number is only meaningful in the context of the assay that produced it, and the total IgA still has to be adequate for the tTG-IgA to be trusted at all. Recent analysis of how total IgA levels influence non-biopsy diagnosis reinforces that the antibody value cannot be read in isolation.

What the biopsy still settles

Upper endoscopy with several biopsies of the duodenum remains the confirmation step for most adults, and it does work that serology alone cannot. It grades the degree of villous atrophy, identifies seronegative disease in a person whose antibodies are equivocal, and helps rule out conditions that can mimic celiac disease under the microscope. The biopsy carries the same precondition as the blood work: it is only reliable while the patient is eating gluten.

Where guesswork still creeps in

Two habits undermine an otherwise clean workup. The first is diagnosing celiac disease from symptoms and a positive response to a gluten-free diet alone, which the evidence does not support and which forecloses accurate testing later. The second is misreading genetic testing. Absence of the HLA-DQ2 and HLA-DQ8 genes is useful because it makes celiac disease very unlikely, but their presence proves little, since a large share of the general population carries these genes without ever developing the disease. Genetic testing rules the diagnosis out, not in.

This article is educational and is not medical advice; decisions about testing belong with a qualified clinician. The reassuring part is that the sequence is well defined: keep gluten in the diet, start with tTG-IgA and total IgA, and reserve the no-biopsy path for the specific high-antibody situations the guideline describes.