How do you interpret celiac disease laboratory results, including tissue transglutaminase antibody (tTGA), endomysial antibody (EMA), and deamidated gliadin peptide (DGP) tests, in a patient with suspected celiac disease?

How to Interpret Celiac Disease Laboratory Results

Start with IgA tissue transglutaminase antibody (tTG-IgA) as your first-line screening test, always measure total IgA simultaneously to avoid missing IgA deficiency, and use IgA endomysial antibody (EMA) as confirmatory testing when tTG-IgA is elevated. 1

Initial Testing Algorithm

First-Line Screening

• Order tTG-IgA as the primary screening test, which has 90-96% sensitivity and >95% specificity in adults at the 15 U/mL threshold 1
• Always measure total IgA levels simultaneously to identify IgA deficiency, which occurs in 1-3% of celiac patients and causes falsely negative IgA-based tests 2, 1
• Ensure the patient is consuming at least 10g of gluten daily for 6-8 weeks before testing to avoid false-negative results 1

Interpreting tTG-IgA Results

Positive tTG-IgA (above laboratory threshold):

• Order IgA endomysial antibody (EMA) as confirmatory testing, which has excellent specificity of 99.6% in adults 2, 1
• If tTG-IgA is >10× the upper limit of normal, this correlates strongly with severe intestinal damage and has >98% positive predictive value for celiac disease 3
• The combination of positive tTG-IgA and positive EMA has virtually 100% positive predictive value for celiac disease 3

Negative tTG-IgA with normal total IgA:

• This effectively rules out celiac disease with a negative predictive value of virtually 100% 4
• No further celiac-specific workup is needed unless clinical suspicion remains extremely high 3

Negative tTG-IgA with low or absent total IgA:

• This indicates IgA deficiency, making IgA-based tests unreliable 2, 1
• Switch to IgG-based testing: order IgG deamidated gliadin peptide (DGP-IgG), which has superior diagnostic accuracy (93.6% sensitivity, 99.4% specificity) compared to tTG-IgG 2, 1

Understanding Deamidated Gliadin Peptide (DGP) Testing

• DGP antibodies have reasonably high accuracy but are inferior to tTG-IgA for routine screening 2
• DGP-IgG is the preferred IgG test in IgA-deficient patients, with 93.6% sensitivity and 99.4% specificity 3
• In children under 2 years, combine tTG-IgA with IgG and IgA DGP for improved sensitivity 1
• DGP may detect persistent mucosal damage on gluten-free diet better than tTG or EMA 5

Handling Discordant Results

Positive tTG-IgA but negative EMA:

• This may represent early or developing celiac disease 3
• Proceed directly to upper endoscopy with duodenal biopsy, as this remains the gold standard for diagnosis 3
• Obtain at least 6 biopsy specimens from the second part of duodenum or beyond 3, 1

Isolated positive tTG-IgG with negative tTG-IgA:

• First, verify total IgA levels 3
• If total IgA is normal, the negative tTG-IgA effectively excludes celiac disease in most cases 3
• If IgA deficiency is confirmed, proceed with IgG-based testing (DGP-IgG) 3

Role of HLA-DQ2/DQ8 Testing

• HLA-DQ2 and HLA-DQ8 are present in almost all celiac patients, with >99% negative predictive value when both are absent 2, 1
• Reserve HLA testing for second-line evaluation in cases with inconclusive antibodies, equivocal histology, or patients already on gluten-free diet without prior testing 2, 1
• HLA testing has no additional diagnostic value in cases with high tTG-IgA levels 2
• A gene dose effect exists: homozygous HLA-DQ2.5 makes celiac disease likely, while heterozygous HLA-DQ2.2 makes it unlikely 2

When to Proceed to Biopsy

Mandatory biopsy indications:

• Any positive tTG-IgA result in adults, regardless of EMA status 3, 1
• Discordant serology results (positive tTG-IgA with negative EMA) 3
• High clinical suspicion despite negative serology, especially in high-risk populations (first-degree relatives, type 1 diabetes, autoimmune thyroid disease) 3

Biopsy technique:

• Obtain at least 6 duodenal biopsy specimens 3, 1
• Include 1-2 samples from duodenal bulb and at least 4 from distal duodenum 1
• Ensure proper orientation for histologic analysis using Marsh classification 1

Special Populations

Children:

• tTG-IgA has 97.7% sensitivity and 70.2% specificity at 20 U/mL threshold 2
• In children with tTG-IgA ≥10× upper limit of normal, positive EMA, and positive HLA-DQ2/DQ8, biopsy may be avoided per pediatric guidelines 1
• Add DGP testing in children under 2 years 1

IgA-deficient patients:

• Use IgG-based tests exclusively: DGP-IgG and tTG-IgG 1
• DGP-IgG is superior to tTG-IgG for diagnosis 3

Critical Pitfalls to Avoid

• Never start a gluten-free diet before completing diagnostic testing, as this leads to false-negative serology and inconclusive biopsies 1
• Never rely solely on serology without biopsy confirmation in adults, especially with discordant results 3, 1
• Never use point-of-care tests as definitive diagnostic tools due to quality control issues with inexperienced users 2
• Never confuse elevated total IgA with celiac disease risk—elevated total IgA actually strengthens confidence in negative tTG-IgA results 3
• Never diagnose based on symptom improvement with gluten-free diet alone, as this cannot differentiate celiac disease from non-celiac gluten sensitivity 3

Follow-Up Testing After Diagnosis

• Repeat tTG-IgA at 6 months after starting gluten-free diet, again at 12 months, then annually 3, 1
• Persistently positive serology indicates ongoing gluten exposure or poor dietary compliance 3
• Negative serology after treatment does not guarantee mucosal healing—consider follow-up biopsy in 1-3 years, especially with severe initial presentation 3, 1