How do I diagnose celiac disease in a patient who is currently consuming an unrestricted gluten diet (≥10 grams per day for at least 4–6 weeks) and presents with chronic diarrhea, weight loss, abdominal pain, iron‑deficiency anemia, dermatitis herpetiformis, growth failure, or has a relevant family history?

How to Diagnose Celiac Disease

Begin with IgA tissue transglutaminase antibody (tTG-IgA) testing plus total IgA level measurement as your first-line screening approach, then confirm positive results with upper endoscopy and at least 6 duodenal biopsies showing villous atrophy while the patient is consuming adequate gluten (≥10 grams daily for 6–8 weeks). 1, 2

Pre-Test Requirements: Ensure Adequate Gluten Intake

• The patient must consume at least 10 grams of gluten daily for 6–8 weeks before any serologic or histologic testing to avoid false-negative results. 3, 1
• If the patient has already reduced or eliminated gluten, reintroduce it (equivalent to three slices of wheat bread daily) for 1–3 months before proceeding with diagnostic testing. 1
• Insufficient gluten exposure is the most common cause of false-negative serology and inconclusive biopsies. 3, 1

Step 1: Initial Serologic Screening

Order IgA tissue transglutaminase (tTG-IgA) plus total IgA level simultaneously. 1, 2

• tTG-IgA demonstrates 90.7% sensitivity and 87.4% specificity in adults at the 15 U/mL threshold. 1, 2
• In children, tTG-IgA shows 97.7% sensitivity and 70.2% specificity at the 20 U/mL threshold. 1
• Measuring total IgA is mandatory because selective IgA deficiency occurs in 1–3% of celiac patients and causes falsely negative IgA-based antibody tests. 1, 2, 4

If Total IgA is Low or Absent (IgA Deficiency Detected):

• Switch to IgG-based testing: order IgG deamidated gliadin peptide (DGP-IgG), which has superior accuracy (93.6% sensitivity, 99.4% specificity) compared to IgG tTG. 1
• IgG endomysial antibody (EMA) is also highly effective in IgA-deficient patients. 1
• Do not use IgG tTG as a primary screen when total IgA is normal—it lacks specificity and generates frequent false positives. 1

Step 2: Confirmatory Serologic Testing

If tTG-IgA is positive, confirm with IgA endomysial antibody (EMA) testing. 1

• EMA provides superior specificity: 99.6% in adults and 93.8% in children. 1
• When tTG-IgA exceeds 10 times the upper limit of normal and EMA is positive on a repeat sample, the positive predictive value for celiac disease approaches 100%. 1

Step 3: Endoscopic Confirmation with Duodenal Biopsy

Upper endoscopy with duodenal biopsy is mandatory in adults to confirm the diagnosis—serology alone cannot replace biopsy except in patients with coagulation disorders or pregnancy. 2

Biopsy Technique:

• Obtain at least 6 duodenal biopsy specimens: 1–2 from the duodenal bulb and a minimum of 4 from the second portion of the duodenum or beyond, because mucosal changes can be patchy. 1, 2
• Request proper specimen orientation for histologic analysis using the Marsh or Marsh-Oberhuber classification system. 3, 1

Histologic Criteria for Definitive Diagnosis:

• Villous atrophy (Marsh type 3) with crypt hyperplasia and ≥25 intraepithelial lymphocytes per 100 enterocytes establishes the diagnosis. 2
• Probable celiac disease may be diagnosed when biopsies show ≥25 intraepithelial lymphocytes without villous atrophy, combined with positive serology. 2

Step 4: HLA Typing (Selective Use Only)

HLA-DQ2 and HLA-DQ8 testing has a negative predictive value >99%—absence of both alleles essentially rules out celiac disease. 1, 2

When to Order HLA Testing:

• Equivocal small intestine histology findings. 2
• Patient already on a gluten-free diet before testing (though gluten challenge is preferred). 2
• Discordant serology and histology results. 2
• HLA typing should not be used as a primary diagnostic tool, only to exclude disease in uncertain cases. 3, 1

Critical Diagnostic Pitfalls to Avoid

• Never diagnose celiac disease based on serology alone without biopsy confirmation in adults. 1, 2
• Never initiate a gluten-free diet before completing the full diagnostic workup—this invalidates subsequent serologic and histologic testing. 3, 1
• Do not rely on symptoms alone or symptom response to a gluten-free diet, as this cannot differentiate celiac disease from non-celiac gluten sensitivity. 2
• Avoid ordering multiple antibody panels simultaneously in low-risk individuals, as this reduces specificity without meaningful gains in sensitivity. 1

Tests That Should NOT Be Ordered

• Small bowel follow-through imaging. 1
• Intestinal permeability testing. 1
• D-xylose testing. 1
• Salivary antibody testing. 1
• Non-deamidated IgA/IgG gliadin antibody assays (older-generation tests). 1

Special Populations and High-Risk Groups

Consider celiac disease screening in patients with: 4, 5

• First-degree relatives of celiac patients (10% prevalence). 4
• Type 1 diabetes (5–10% prevalence in celiac patients). 4
• Autoimmune thyroid disease. 1
• Iron deficiency anemia resistant to oral supplementation (present in 48% of adults with celiac disease at presentation). 4
• Dermatitis herpetiformis (pathognomonic for celiac disease—diagnosed by granular IgA deposits in dermal papillae on direct immunofluorescence). 4, 5
• Unexplained osteoporosis or fractures (fracture risk increases 60–100% before diagnosis). 4
• Unexplained elevated liver enzymes. 4
• Infertility or recurrent miscarriages. 4

Seronegative Celiac Disease

If clinical suspicion remains high despite negative tTG-IgA: 1

• Verify adequate gluten consumption (≥10 g daily for 6–8 weeks). 1
• Confirm total IgA was measured to exclude IgA deficiency. 1
• Proceed to upper endoscopy with at least 6 duodenal biopsies to assess for seronegative celiac disease, which represents a subset of patients with characteristic intestinal damage but negative antibody testing. 1

Post-Diagnosis Management

• Initiate strict lifelong gluten-free diet immediately after biopsy confirmation (daily gluten intake <10 mg). 3, 1
• Refer to a registered dietitian experienced in celiac disease management for comprehensive dietary education. 1, 4
• Repeat tTG-IgA at 6 months after starting gluten-free diet, at 12 months, then annually. 1
• Persistently positive serology indicates ongoing intestinal damage and gluten exposure. 1