Diagnostic Testing for Celiac Disease
The first-line test to confirm celiac disease is IgA tissue transglutaminase antibody (tTG-IgA) combined with total IgA level measurement, followed by upper endoscopy with at least 6 duodenal biopsies showing villous atrophy—serology alone cannot replace biopsy in adults. 1, 2
Initial Serologic Testing
Start with IgA tTG plus total IgA measurement while the patient consumes at least 10g of gluten daily for 6-8 weeks. 1, 2 This combination is the most efficient screening approach because:
- IgA tTG demonstrates 90.7% sensitivity and 87.4% specificity in adults at 15 U/mL threshold 1, 2
- Total IgA measurement identifies IgA deficiency, which occurs in 1-3% of celiac patients and causes falsely negative IgA-based tests 1, 2
- Testing without adequate gluten intake produces false-negative results 1, 2
Confirmatory Serologic Testing
When IgA tTG is positive, obtain IgA endomysial antibody (EMA) for confirmation. 1, 2 EMA provides superior specificity of 99.6% in adults, and when both tTG exceeds 10 times the upper limit of normal and EMA is positive on repeat testing, the positive predictive value approaches 100%. 1
Special Population: IgA Deficiency
If total IgA is low or absent, switch to IgG-based testing with IgG deamidated gliadin peptide (DGP-IgG). 1 DGP-IgG demonstrates 93.6% sensitivity and 99.4% specificity in adults with IgA deficiency. 1 Do not use IgG tTG as a primary screen when total IgA is normal, as it lacks adequate specificity in that setting. 1
Mandatory Histologic Confirmation
Upper endoscopy with duodenal biopsy remains essential for diagnosis in adults and cannot be replaced by serology alone. 3, 2 The only exceptions are patients with coagulation disorders or pregnancy. 3
Biopsy Protocol
- Obtain at least 6 specimens: 1-2 from the duodenal bulb and at least 4 from the second portion of the duodenum or beyond 1, 2
- Multiple biopsies are necessary because mucosal changes can be patchy 2
- Definitive diagnosis requires villous atrophy (Marsh 3 lesion) with crypt hyperplasia and increased intraepithelial lymphocytes (≥25 IELs per 100 enterocytes) 3, 2
Lesser degrees of damage (≥25 IELs without villous atrophy) combined with positive serology may indicate probable celiac disease. 3
Role of Genetic Testing
HLA-DQ2/DQ8 testing should not be performed routinely but is useful in select scenarios with equivocal histology or discordant serology. 1, 2 The negative predictive value exceeds 99% when both DQ2 and DQ8 are absent, effectively ruling out celiac disease. 1, 2 However, 25-30% of the white population carries HLA-DQ2, making it unsuitable as a primary diagnostic test. 2
When to Consider HLA Testing
- Equivocal small intestinal histology findings 1
- Patient already on gluten-free diet before testing 1
- Discordant serology and histology results 1
Critical Pitfalls to Avoid
Never start a gluten-free diet before completing the diagnostic workup. 2 This leads to false-negative serology and inconclusive biopsies. 2 If a patient has already reduced gluten intake, they must resume at least 10g daily (approximately three slices of wheat bread) for 1-3 months before repeat testing. 1
Never diagnose celiac disease based on serology alone without biopsy confirmation in adults. 3, 2 Even with highly elevated antibody levels, biopsy remains mandatory to establish the diagnosis. 3
Verify adequate gluten consumption at the time of testing. 1, 2 Patients restricting gluten intake will have falsely negative results regardless of test sensitivity. 1
Algorithm Summary
- Confirm gluten intake: At least 10g daily for 6-8 weeks 1, 2
- Order IgA tTG + total IgA 1, 2
- If IgA tTG positive: Add IgA EMA for confirmation 1
- If total IgA deficient: Switch to IgG DGP 1
- Proceed to endoscopy with ≥6 duodenal biopsies regardless of antibody levels 3, 2
- Confirm villous atrophy on histology for definitive diagnosis 3, 2
- Consider HLA-DQ2/DQ8 only if results are equivocal or discordant 1, 2