What is tTG Blood Testing?
tTG (tissue transglutaminase) blood testing is a highly sensitive and specific serological test that measures IgA antibodies against tissue transglutaminase, the primary autoantigen in celiac disease, and serves as the preferred first-line screening test for detecting this immune-mediated disorder. 1
Test Mechanism and Purpose
tTG testing detects circulating IgA autoantibodies to tissue transglutaminase-2 (TG2), an enzyme that becomes the target of autoimmune attack in celiac disease. When individuals with genetic susceptibility (HLA-DQ2 or HLA-DQ8) consume gluten, TG2 deamidates gliadin peptides, increasing their immunogenicity and triggering antibody production 2.
The IgA tTG test demonstrates specificity greater than 95% and sensitivity in the range of 90-96%, making it the most efficient single serologic test for celiac disease detection 1.
Clinical Application
When to Order the Test
tTG testing should be performed in:
- Symptomatic patients with gastrointestinal complaints (diarrhea, abdominal pain, weight loss) or extraintestinal manifestations (fatigue, anemia, dermatitis herpetiformis) 2, 3
- High-risk populations including patients with type 1 diabetes (5.4% prevalence), autoimmune thyroid disease, or first-degree relatives of celiac patients 2, 4
- Children with type 1 diabetes soon after diagnosis, with repeat screening at 2 and 5 years 4
Critical Testing Considerations
A quantitative serum IgA level must be obtained simultaneously with tTG testing 2. This is essential because:
- IgA deficiency occurs in 1 in 500 people generally, but in 1-3% of celiac patients
- IgA deficiency causes falsely low IgA tTG results
- If IgA deficient, use IgG-based tests (IgG tTG or IgG EMA) instead 1
Interpretation of Results
Positive Results
Patients must remain on a gluten-containing diet during testing and until biopsy is completed, as gluten avoidance reduces antibody levels and may cause false-negative results 1.
- Very elevated levels (≥10× upper limit of normal): Positive predictive value of 91-95% for biopsy-confirmed celiac disease 5, 6, 7
- Low to moderately positive levels: Require clinical context interpretation and often repeat testing every 6-12 months 2
- All positive results should be confirmed before proceeding to gastroenterology referral 2
Negative Results
A negative tTG test has excellent negative predictive value (approaching 100%), making celiac disease unlikely 1. However, if clinical suspicion remains high despite negative serology, consider:
- Checking serum IgA level (may have been deficient)
- HLA-DQ2/DQ8 genetic testing
- Proceeding directly to endoscopy with duodenal biopsy 1
Important Caveats
Test Performance Variability
Significant variability exists among commercial laboratories in North America, with sensitivity ranging from 40% to 86.4% and specificity from 41.7% to 100% between different labs 8. The positive predictive value varies from 71.5% to 88.8% depending on the assay used 6.
False Positives
Approximately 5-10% of patients with tTG ≥10× ULN do not have histologic celiac disease on biopsy 6. False positives are more common in:
- Patients with type 1 diabetes (PPV drops to 89%) 6, 7
- Those with other autoimmune conditions
- Patients with inflammatory bowel disease 9
Sensitivity Limitations
Sensitivity is significantly lower (42.3%) in patients with partial villous atrophy (Marsh IIIA) compared to 90% in those with total villous atrophy 8. This means tTG testing may miss milder cases of celiac disease.
Diagnostic Algorithm
- Screen with IgA tTG + total serum IgA in symptomatic or high-risk patients
- If IgA deficient: Use IgG-based tests (IgG tTG or IgG EMA)
- If tTG positive: Confirm with repeat testing or EMA
- If confirmed positive: Refer to gastroenterology for endoscopy with duodenal biopsy (remains gold standard for diagnosis) 2, 1
- Do not start gluten-free diet until after biopsy confirmation, as this prevents accurate diagnosis 1
The tTG test is not diagnostic by itself—small bowel biopsy showing villous atrophy remains the gold standard for confirming celiac disease in adults 1, 10. The exception is in some pediatric protocols where very high tTG levels (≥10× ULN) with positive confirmatory testing may allow biopsy avoidance 11, 4.