How is celiac disease (CD) tested?

How Celiac Disease is Tested

The diagnostic approach begins with IgA tissue transglutaminase antibody (tTG-IgA) testing while the patient is on a gluten-containing diet, followed by upper endoscopy with multiple duodenal biopsies for histologic confirmation. 1

Initial Serologic Testing

The tTG-IgA test is the preferred first-line screening test with sensitivity of 90-96% and specificity exceeding 95%. 2, 1, 3

Critical Pre-Test Requirements:

• Testing MUST be performed while the patient is consuming gluten (at least 10g daily for 6-8 weeks). 1, 4 Starting a gluten-free diet before testing leads to false-negative results and is a major diagnostic pitfall. 2, 4
• Measure total IgA levels simultaneously to rule out IgA deficiency, which affects approximately 2-3% of celiac patients and causes false-negative IgA-based tests. 2, 1, 4

Confirmatory Serology:

• When tTG-IgA is >10× the upper limit of normal, obtain IgA endomysial antibody (EMA-IgA) as a confirmatory test with excellent specificity of 99.6%. 2, 1, 4
• The combination of very high tTG-IgA plus positive EMA-IgA yields a positive predictive value approaching 100% for celiac disease. 2, 1

Biopsy Confirmation (Gold Standard)

Upper endoscopy with duodenal biopsies remains the diagnostic gold standard in adults, despite high serologic accuracy. 2, 1, 4

Proper Biopsy Technique:

• Obtain at least 6 biopsy specimens total: 1-2 from the duodenal bulb and at least 4 from the second part of the duodenum or beyond. 2, 1, 4
• Multiple biopsies are essential because celiac disease can have patchy distribution. 2

Histologic Findings:

• Characteristic changes include villous atrophy, crypt lengthening, and increased intraepithelial lymphocytes. 1, 4, 3
• Important caveat: Lymphocytic infiltration alone without villous atrophy is NOT specific for celiac disease—consider other causes like H. pylori infection, bacterial overgrowth, or autoimmune disorders. 2

Special Testing Scenarios

IgA-Deficient Patients:

• Use IgG-based tests instead: IgG deamidated gliadin peptide (DGP-IgG) or IgG tissue transglutaminase (tTG-IgG). 2, 1, 4
• Critical warning: IgG isotype testing for tTG is NOT specific in patients with normal IgA levels and should not be used in that setting. 2, 4

Children Under 2 Years:

• Combine tTG-IgA with both IgG and IgA deamidated gliadin peptides to improve sensitivity in this age group. 2, 1, 4

Biopsy-Avoidance Protocol (Pediatrics Only):

• In children with tTG-IgA ≥10× upper limit of normal, positive EMA-IgA, AND positive HLA-DQ2/DQ8, biopsy may be avoided. 1, 4
• This approach is NOT standard for adults, who should still undergo biopsy for differential diagnosis. 2

HLA Genetic Testing

HLA-DQ2 and HLA-DQ8 testing has limited diagnostic utility but excellent negative predictive value (>99%). 2, 1, 4

When to Use HLA Testing:

• When celiac disease is strongly suspected despite negative serology 2, 1, 3
• In patients with equivocal biopsy findings 2
• In patients already on a gluten-free diet who were never properly tested 2, 1
• In patients with Down syndrome or other scenarios requiring rule-out 2

If both HLA-DQ2 and HLA-DQ8 are absent, celiac disease is essentially ruled out. 2, 4

Diagnostic Algorithm for Negative Serology with High Suspicion

When serology is negative but clinical suspicion remains high:

1. Confirm adequate gluten intake (patient may have self-restricted gluten) 2
2. Verify total IgA level is normal (rule out IgA deficiency) 2, 1
3. Perform HLA-DQ2/DQ8 testing—if positive, proceed to biopsy 1
4. If suspicion remains very high, perform biopsy regardless of negative serology 2

Testing After Gluten-Free Diet Started (Common Error)

If a patient has already started a gluten-free diet before testing:

• Resume normal diet with three slices of wheat bread daily for 1-3 months before repeat testing. 2, 1
• Some patients may require several years of gluten challenge for histologic relapse to occur. 1
• This is a major diagnostic pitfall that significantly complicates diagnosis. 2, 4

Obsolete Tests to Avoid

• Antibodies to native gliadin (AGA) are no longer recommended for primary detection due to poor sensitivity and specificity. 2
• Non-deamidated IgA/IgG gliadin antibodies confer no additional diagnostic benefit in adults. 5

Follow-Up Testing After Diagnosis

• Perform follow-up serology at 6 months, 12 months, then yearly to monitor adherence to gluten-free diet. 2, 1, 4
• Critical limitation: Negative serology does NOT guarantee intestinal mucosal healing—44% of patients with persistent villous atrophy may have normal tTG levels. 2, 5
• Persistently positive serology usually indicates ongoing gluten exposure and intestinal damage. 2, 1, 4
• For patients with persistent or relapsing symptoms, perform repeat endoscopy with biopsies even if serology is negative. 2, 1

High-Risk Groups Requiring Testing

Test the following populations even without classic symptoms:

• First-degree relatives of celiac patients 1, 3
• Unexplained iron deficiency anemia 2, 1, 3
• Type 1 diabetes mellitus 1, 3
• Premature osteoporosis 1, 3
• Autoimmune thyroid disease 1, 3
• Unexplained elevated liver transaminases 1, 3
• Down syndrome 2, 1, 3
• Symptoms of malabsorption (chronic diarrhea with weight loss, steatorrhea, postprandial bloating) 2