What laboratory tests are recommended for diagnosing celiac disease?

Laboratory Testing for Celiac Disease

The IgA tissue transglutaminase antibody (tTG-IgA) is the single best initial screening test for celiac disease and should be ordered with a total IgA level simultaneously. 1, 2

Initial Serological Testing

Order these two tests together as your first step:

• IgA tissue transglutaminase antibody (tTG-IgA) - This has 90-96% sensitivity and >95% specificity, making it the preferred first-line test 3, 1
• Total IgA level - This must be measured simultaneously to identify IgA deficiency, which occurs in 1-3% of celiac patients and causes false-negative tTG-IgA results 3, 1, 2

Critical requirement: Testing must be performed while the patient consumes at least 10g of gluten daily (approximately 3 slices of wheat bread) for 6-8 weeks. Starting a gluten-free diet before testing leads to false-negative results. 1, 2, 4

Confirmatory Serological Testing

If tTG-IgA is elevated (especially >10× upper limit of normal):

• Order IgA endomysial antibody (EMA-IgA) as confirmatory testing with excellent specificity of 99.6% 3, 1
• The combination of tTG-IgA >10× upper limit of normal plus positive EMA-IgA approaches 100% positive predictive value for celiac disease 2, 4

Testing in IgA-Deficient Patients

If total IgA is low or deficient:

• Switch to IgG-based tests: IgG tissue transglutaminase (tTG-IgG) and/or IgG deamidated gliadin peptide (DGP-IgG) 3, 1, 4
• Do NOT use IgG-based tests in patients with normal IgA levels, as they are markedly less accurate in that setting 4

Special Population Considerations

In children under 2 years:

• Combine tTG-IgA with both IgG and IgA deamidated gliadin peptides to improve sensitivity 3, 2, 4

Genetic Testing (Selective Use Only)

HLA-DQ2 and HLA-DQ8 testing should be ordered in specific scenarios only:

• When celiac disease is strongly suspected despite negative serology 3, 1, 2
• In patients with equivocal biopsy findings 3, 2, 4
• In patients already on a gluten-free diet who were never properly tested 2, 4
• The negative predictive value is >99% - absence of both alleles essentially rules out celiac disease 3, 1, 2

Do NOT order HLA testing routinely or when tTG-IgA is highly elevated, as it adds no diagnostic value in those scenarios. 2

Tests That Should NOT Be Ordered

• IgA or IgG antigliadin antibodies - These are outdated and no longer recommended for primary detection due to poor diagnostic performance compared to tTG-IgA and EMA 3
• Combining multiple serological tests instead of tTG-IgA alone reduces specificity and is not recommended in low-risk populations 3

Biopsy Confirmation (Not a "Lab" but Essential)

While not a laboratory test, upper endoscopy with multiple duodenal biopsies remains mandatory in adults despite positive serology:

• 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 3, 2, 4
• Biopsy establishes definitive diagnosis and rules out other causes of villous atrophy 2
• Characteristic histologic findings include villous atrophy, crypt lengthening, and increased intraepithelial lymphocytes 3, 1, 4

Common Pitfalls to Avoid

• Never start a gluten-free diet before completing diagnostic testing - This is the most common error and leads to false-negative results 1, 4
• Do not rely solely on serology without biopsy confirmation in adults - This can lead to misdiagnosis 3, 4
• Do not use symptom improvement on a gluten-free diet as diagnostic evidence - This has very low positive predictive value for celiac disease 3, 1
• Do not assume negative serology after treatment means mucosal healing - Serology can normalize while villous atrophy persists 4, 5

Follow-Up Laboratory Testing

After diagnosis and treatment initiation:

• Repeat tTG-IgA at 6 months, 12 months, and yearly thereafter 1, 4
• Persistently positive serology usually indicates ongoing gluten exposure 1, 4
• However, serology has only 50% sensitivity for detecting persistent villous atrophy in patients on a gluten-free diet, so negative antibodies do not guarantee mucosal healing 5