When standard IBS treatments are not working, the problem may not be your treatment plan. It may be your diagnosis. Celiac disease is one of the most common conditions hiding behind an IBS label, affecting roughly 1 in 100 people worldwide with up to 83% of cases undiagnosed. The good news is that celiac disease has specific, accurate tests. This article walks through every test available, explains what each one measures, covers the situations where results can be misleading, and provides a practical roadmap for getting tested.
When should you consider celiac testing?
The American College of Gastroenterology recommends celiac screening for patients with chronic diarrhea, and many gastroenterologists extend that recommendation to anyone with persistent IBS symptoms that have not responded to first-line treatments. If you have been following a low-FODMAP diet, taking antispasmodics, trying fiber supplementation, or using neuromodulators without meaningful improvement, celiac disease should be on the list of conditions to rule out.
Red flags that should prompt celiac testing in IBS patients
- IBS symptoms that have not improved after 3 to 6 months of guideline-based treatment
- Iron deficiency anemia, especially if it does not respond to oral iron
- Unexplained weight loss or failure to gain weight despite adequate calorie intake
- A first-degree relative with confirmed celiac disease
- A personal history of another autoimmune condition (type 1 diabetes, autoimmune thyroid disease, autoimmune liver disease)
- Dermatitis herpetiformis (itchy blistering rash on elbows, knees, or buttocks)
- Unexplained osteoporosis or osteopenia, particularly if diagnosed before age 50
- Recurrent mouth ulcers, dental enamel defects, or unexplained elevated liver enzymes
What blood tests are used for celiac disease?
Several antibody tests are available for celiac disease, but they are not interchangeable. Each has specific strengths, limitations, and clinical indications. The following section covers the main tests in the order they are typically used.
tTG-IgA: the first-line screening test
Tissue transglutaminase IgA (tTG-IgA) is the recommended first-line blood test for celiac disease by the American College of Gastroenterology, the British Society of Gastroenterology, and the European Society for Paediatric Gastroenterology, Hepatology and Nutrition. It measures IgA antibodies directed against the enzyme tissue transglutaminase, which is involved in the immune response to gluten in celiac disease. In patients with normal IgA levels who are actively eating gluten, the tTG-IgA test has sensitivity above 95% and specificity above 95% (Husby et al., 2012). This makes it one of the most accurate screening tests in gastroenterology.
â ī¸tTG-IgA is only accurate when you are eating gluten. If you have reduced or eliminated gluten from your diet, antibody levels will drop regardless of whether you have celiac disease. You need to be consuming at least 3 grams of gluten daily (roughly 2 slices of standard wheat bread) for a minimum of 6 weeks before testing.
Total serum IgA: why it must be co-ordered
Total serum IgA should always be measured alongside tTG-IgA. The reason is straightforward: approximately 2 to 3% of celiac patients have selective IgA deficiency, a condition in which the body produces very little or no IgA antibody. This is roughly 10 to 15 times more common in celiac patients than in the general population (Chow et al., 2012). If your total IgA is low, any IgA-based celiac test (tTG-IgA, EMA-IgA) will produce a falsely negative result regardless of whether celiac disease is present.
DGP antibodies: the alternative for IgA-deficient patients
Deamidated gliadin peptide (DGP) antibodies, particularly DGP-IgG, are the recommended alternative when IgA deficiency is identified. Unlike older gliadin antibody tests (which had poor specificity), DGP tests target specific deamidated peptides of gliadin that are more immunogenic and more specific to celiac disease. DGP-IgG has sensitivity of approximately 80 to 98% depending on the assay and study population (Volta et al., 2010). Some laboratories offer a combined DGP-IgG and tTG-IgG panel for IgA-deficient patients.
EMA: the high-specificity confirmatory test
Endomysial antibody (EMA) IgA testing has specificity approaching 99 to 100% for celiac disease. It detects antibodies against the endomysium (the connective tissue around muscle fibers) using immunofluorescence. Because of its high specificity, EMA is sometimes used as a confirmatory test when tTG-IgA is positive but the result is equivocal or only mildly elevated. However, EMA is more expensive, operator-dependent, and slightly less sensitive than tTG-IgA, which is why it is not the first-line test.
What is the gluten challenge, and do you need one?
The gluten challenge is a period of intentional gluten consumption before celiac testing. It is required only if you have already reduced or eliminated gluten from your diet. If you are still eating a normal gluten-containing diet, no challenge is necessary, and you can proceed directly to testing.
The standard gluten challenge protocol involves consuming at least 3 grams of gluten per day (approximately the amount in 2 slices of standard wheat bread) for a minimum of 6 to 8 weeks before blood testing, or for at least 2 weeks before endoscopy with biopsy (Leffler et al., 2013). Some patients experience significant symptom flares during the challenge. If symptoms are intolerable, the challenge can sometimes be shortened to 2 weeks if biopsy is planned, but this should be decided with your gastroenterologist.
âšī¸If you went gluten-free on your own and felt better, that is worth noting, but it does not diagnose celiac disease. Many conditions improve with gluten removal, including non-celiac gluten sensitivity and FODMAP sensitivity (wheat is a high-FODMAP food). Only celiac-specific testing while eating gluten can distinguish between these possibilities.
Can genetic testing rule out celiac disease?
HLA-DQ2 and HLA-DQ8 are human leukocyte antigen (HLA) gene variants that are necessary, but not sufficient, for celiac disease to develop. Approximately 95% of celiac patients carry HLA-DQ2, and most of the remaining 5% carry HLA-DQ8. Together, these two alleles are present in roughly 99% of confirmed celiac cases (Sollid et al., 2012).
The clinical value of genetic testing lies in its negative predictive value, which is nearly 100%. If you are negative for both HLA-DQ2 and HLA-DQ8, you almost certainly do not have celiac disease and will not develop it in the future. This makes genetic testing particularly useful in two situations: ruling out celiac in patients who are already on a gluten-free diet (where antibody tests are unreliable), and screening family members of celiac patients. If a family member tests negative for both alleles, they can be reassured that they do not carry the genetic prerequisite.
However, genetic testing cannot confirm celiac disease. Approximately 30 to 40% of the general population carries HLA-DQ2 or HLA-DQ8, and only about 3% of those individuals will ever develop celiac disease. A positive genetic test means celiac is possible, not that it is present. Diagnosis still requires serological and/or histological confirmation.
What happens during endoscopy and biopsy?
Upper endoscopy (esophagogastroduodenoscopy, or EGD) with duodenal biopsy is the gold standard for confirming celiac disease. The procedure involves passing a flexible tube with a camera through the mouth into the duodenum (the first part of the small intestine), where tissue samples are taken. Current guidelines recommend at least 4 biopsies from the second part of the duodenum (D2) and at least 1 to 2 from the duodenal bulb (D1), because celiac damage can be patchy.
Biopsy findings are classified using the Marsh classification (modified by Oberhuber et al., 1999). Marsh 0 is normal mucosa. Marsh 1 shows increased intraepithelial lymphocytes (IELs) only. Marsh 2 adds crypt hyperplasia. Marsh 3 shows villous atrophy, which is subdivided into partial (3a), subtotal (3b), and total (3c). Marsh 3 in the presence of positive celiac serology is considered diagnostic. Marsh 1 or 2 findings require careful clinical correlation, as they can occur in other conditions.
Marsh classification summary
- Marsh 0: Normal mucosa, no celiac disease
- Marsh 1: Increased intraepithelial lymphocytes (above 25 per 100 epithelial cells). Non-specific; can occur in infections, NSAID use, and other conditions
- Marsh 2: Increased IELs plus crypt hyperplasia. Suggestive but not conclusive for celiac disease
- Marsh 3a: Partial villous atrophy. With positive serology, this confirms celiac disease
- Marsh 3b: Subtotal villous atrophy. Confirms celiac disease
- Marsh 3c: Total villous atrophy. Confirms celiac disease
What if test results are uncertain?
Diagnostic uncertainty occurs in several scenarios. A mildly elevated tTG-IgA (less than 2 times the upper limit of normal) with a normal biopsy may represent very early celiac disease, non-celiac gluten sensitivity, or a false-positive result. In such cases, EMA testing can help clarify: if EMA is positive, celiac is very likely; if negative, the tTG-IgA elevation may be non-specific.
Seronegative celiac disease (negative serology, positive biopsy) is another area of uncertainty. If biopsy shows villous atrophy but antibodies are negative, other causes of villous atrophy must be excluded. These include tropical sprue, autoimmune enteropathy, common variable immunodeficiency, Crohn's disease of the small intestine, and medication-related injury (particularly olmesartan). HLA-DQ2/DQ8 testing can be helpful in this context: if the genetic markers are absent, celiac is essentially excluded and another cause of villous atrophy should be pursued.
For patients with borderline results, the clinical approach often involves close monitoring. Repeat serology in 6 to 12 months while maintaining a gluten-containing diet, or proceed to biopsy if clinical suspicion is high. If you are in this uncertain territory, a gastroenterologist with celiac expertise is the right person to guide the next steps.
What comes after a celiac diagnosis?
If testing confirms celiac disease, the treatment is a strict, lifelong gluten-free diet. Most patients see symptom improvement within days to weeks, though complete mucosal healing can take 1 to 2 years in adults. Follow-up typically includes repeat tTG-IgA testing at 6 and 12 months to confirm antibody normalization, a DEXA scan to assess bone density if not already done, evaluation for nutritional deficiencies (iron, folate, B12, vitamin D, calcium, zinc), and referral to a dietitian experienced in the gluten-free diet.
Tracking your dietary response carefully during the transition period can help identify accidental gluten exposures and confirm that your symptoms are genuinely improving. The GLP1Gut app can help you log meals, symptoms, and bowel patterns during this period to build a clear picture of your response to the gluten-free diet.
How accurate is the tTG-IgA blood test for celiac disease?
In patients with normal IgA levels who are actively eating gluten, tTG-IgA has sensitivity above 95% and specificity above 95%. This means it correctly identifies more than 95 out of 100 people who have celiac disease and correctly clears more than 95 out of 100 who do not. Accuracy drops if you are not eating gluten or if you have IgA deficiency.
What is IgA deficiency, and why does it matter for celiac testing?
Selective IgA deficiency means your body produces little or no IgA antibody. It affects about 2 to 3% of celiac patients, roughly 10 times the rate in the general population. It matters because all IgA-based celiac tests will come back falsely negative. This is why total serum IgA should always be ordered alongside tTG-IgA. If IgA is low, your doctor should order IgG-based tests like DGP-IgG instead.
Can a negative genetic test (HLA-DQ2/DQ8) rule out celiac disease?
Effectively, yes. HLA-DQ2 and HLA-DQ8 are present in roughly 99% of celiac patients. If you test negative for both alleles, your chance of having or developing celiac disease is extremely low. This makes genetic testing useful for ruling out celiac in people already on a gluten-free diet, where antibody tests are unreliable.
Do I need to eat gluten before celiac testing?
Yes, if you have already reduced or removed gluten from your diet. The recommendation is at least 3 grams of gluten daily (about 2 slices of wheat bread) for 6 to 8 weeks before blood tests, or at least 2 weeks before biopsy. If you are still eating a normal diet containing gluten, no special preparation is needed.
What does the Marsh classification mean on a biopsy report?
The Marsh classification grades intestinal damage from 0 (normal) to 3c (total villous atrophy). Marsh 3 (any subtype: a, b, or c) combined with positive celiac serology confirms the diagnosis. Marsh 1 and 2 are less specific and can occur in other conditions. Your gastroenterologist will interpret biopsy results in the context of your blood work and symptoms.