What is the appropriate work‑up for a patient suspected of celiac disease while still consuming a gluten‑containing diet?

Work-up for Celiac Disease

Initial Serologic Testing While on Gluten-Containing Diet

The first-line diagnostic test is IgA tissue transglutaminase antibody (tTG-IgA) combined with total IgA level measurement, which must be performed while the patient consumes at least 10g of gluten daily for 6-8 weeks. 1, 2

Critical Pre-Test Requirements

• Verify adequate gluten intake: Patient must consume at least 10g of gluten daily (approximately 3 slices of wheat bread) for 6-8 weeks before testing to avoid false-negative results 1, 2
• Measure total IgA simultaneously: This identifies IgA deficiency, which occurs in 1-3% of celiac patients and causes falsely negative IgA-based tests 1, 2, 3

Test Performance Characteristics

• tTG-IgA demonstrates 90.7% sensitivity and 87.4% specificity in adults at 15 U/mL threshold 4, 1
• In children, tTG-IgA shows 97.7% sensitivity and 70.2% specificity at 20 U/mL threshold 4

Confirmatory Testing Strategy

If tTG-IgA is Positive

Order IgA endomysial antibody (EMA) testing as confirmatory serology because EMA provides superior specificity of 99.6% in adults. 4, 1

• When tTG-IgA exceeds 10 times the upper limit of normal AND a repeat sample is EMA-positive, the positive predictive value approaches 100% 4
• This combination virtually confirms celiac disease and warrants proceeding to biopsy 4

If Total IgA is Low or Absent

Switch to IgG-based testing using IgG deamidated gliadin peptide (DGP-IgG), which demonstrates 93.6% sensitivity and 99.4% specificity in adults. 4, 1

• IgG EMA or IgG tTG can also be used, but DGP-IgG is preferred for superior diagnostic accuracy 4
• Do NOT use IgG tTG as a primary screen when total IgA is normal, as it lacks specificity in that setting 4

Histologic Confirmation

Upper endoscopy with duodenal biopsy is mandatory for diagnosis in adults and cannot be replaced by serology alone, except in patients with coagulation disorders or pregnancy. 1, 5, 2

Biopsy Protocol

• Obtain at least 6 biopsy specimens: 1-2 from the duodenal bulb and minimum of 4 from the second portion of the duodenum or beyond 1, 5
• Multiple biopsies are essential because celiac disease has patchy distribution 1
• Request evaluation by a pathologist with gastroenterology expertise, as poorly oriented mucosa leads to misinterpretation 6

Diagnostic Histologic Criteria

• Definitive diagnosis: Villous atrophy (Marsh type 3) with crypt hyperplasia and ≥25 intraepithelial lymphocytes per 100 enterocytes 1, 5
• Probable celiac disease: ≥25 intraepithelial lymphocytes without villous atrophy, combined with positive serology 5

Genetic Testing (Selective Use Only)

HLA-DQ2/DQ8 testing should NOT be performed routinely but is useful in select scenarios with a negative predictive value exceeding 99% when both are absent. 1, 7

When to Consider HLA Testing

• Patients with equivocal small intestine histology findings 5
• Those already on a gluten-free diet before testing 5
• When serology and histology are discordant 5
• To rule out celiac disease in seronegative patients with villous atrophy 6

Interpretation Caveats

• 95% of celiac patients have HLA-DQ2, and 5% have HLA-DQ8 1
• However, HLA-DQ2 is present in 25-30% of the white population, making it unsuitable as a primary diagnostic test 1
• Negative HLA-DQ2 and DQ8 essentially rules out celiac disease 6, 7

Special Diagnostic Scenarios

If Patient Already Started Gluten-Free Diet

• If gluten-free diet duration is less than one month, serologic and histologic findings are often still abnormal and can be used for diagnosis 6
• If findings are normal on gluten-free diet, gluten challenge testing is the preferred diagnostic test in HLA-DQ2/DQ8-positive patients 6
• Patient must resume 10g gluten daily for 1-3 months before repeat testing 4

If Serology is Negative but Clinical Suspicion Remains High

Proceed directly to upper endoscopy with duodenal biopsies if clinical suspicion of celiac disease remains high despite negative serology. 4, 7

• Verify patient was consuming adequate gluten (≥10g daily for 6-8 weeks) when tested 4
• Confirm total IgA level was measured to exclude IgA deficiency 4
• Consider HLA-DQ2/DQ8 testing to rule out disease if both are absent 6, 4

Common Pitfalls to Avoid

• Never start a gluten-free diet before completing diagnostic workup, as this leads to false-negative serology and inconclusive biopsies 1, 5
• Never diagnose celiac disease based on serology alone without biopsy confirmation in adults 1, 5
• Never rely on symptoms alone or symptom response to gluten-free diet to differentiate celiac disease from non-celiac gluten sensitivity 6, 5
• Do not use IgG tTG as a primary screening test when total IgA is normal 4

Post-Diagnosis Initial Management

Once celiac disease is confirmed by biopsy:

• Initiate strict lifelong gluten-free diet immediately after biopsy confirmation 1, 5
• Refer to a registered dietitian experienced in celiac disease management 6, 1
• Test for micronutrient deficiencies: iron, folic acid, vitamin D, vitamin B12 6
• Repeat tTG-IgA at 6 months after starting gluten-free diet, then at 12 months, and annually thereafter 1