What is a TG-Ab (Tissue Transglutaminase Antibody) Lab Test?
The tissue transglutaminase antibody (TG-Ab or TG2-IgA) test is a blood test that measures IgA antibodies against tissue transglutaminase enzyme, serving as the primary serological screening tool for celiac disease. 1
Primary Clinical Purpose
- TG2-IgA is the crucial first-line serology test for detecting and diagnosing celiac disease in both adults and children 1
- The test identifies autoantibodies produced when individuals with celiac disease consume gluten, as tissue transglutaminase is the main autoantigen recognized in this condition 2
- Sensitivity ranges from 87.5-100% and specificity from 94-98% for detecting untreated celiac disease 3, 4, 5
Diagnostic Thresholds and Interpretation
High-level positivity (>10 times the upper normal limit) is highly reliable for diagnosing active celiac disease:
- When TG2-IgA exceeds 10x the upper limit of normal (ULN), combined with positive endomysial antibody in a second sample, the positive predictive value approaches virtually 100% 1
- At ≥10x ULN threshold, TG2-IgA demonstrates 99.3% specificity and 91.4% positive predictive value 5
- Recent evidence suggests TG2-IgA at ≥10x ULN actually exceeds endomysial antibody testing in specificity 5
- There is a consistent correlation between antibody levels and histological severity of mucosal damage (Marsh grading), with higher antibody levels corresponding to more severe intestinal damage 6
Critical Testing Considerations
Total IgA levels must be checked concurrently, as IgA deficiency causes false-negative results:
- IgA deficiency is an infrequent but important cause of false-negative TG2-IgA testing despite active celiac disease 1
- In IgA-deficient patients, both TG2-IgA and endomysial antibody tests will be negative even with biopsy-proven celiac disease 7
- When IgA deficiency is present, IgG-based testing (TG2-IgG and deamidated gliadin antibody) should be performed instead 1
- IgG isotype testing for TG2 antibody is NOT specific in the absence of IgA deficiency and should not be used routinely 1
Common Pitfalls and Discordant Results
Positive TG2-IgA with negative endomysial antibody results require cautious interpretation:
- This discordance pattern has low probability of true celiac disease (only 17.7% confirmed in one study) 8
- Significant variability exists among different commercial TG2-IgA assay methods, with overall agreement ranging only 28.5-77.7% 8
- Patients positive on multiple different TG2-IgA assays are more likely to have true celiac disease (29.5%) compared to those positive on only one method (2.5%) 8
- False-positive results can occur, particularly in patients with type 1 diabetes mellitus 5
- Discordant results warrant referral to a celiac disease reference center for prolonged follow-up 8
Role in Diagnostic Algorithm
- In patients with strongly positive serology but negative biopsies, TG2-IgA should still be performed, and repeat biopsies may be considered 1
- When celiac disease is first identified by intestinal biopsies, celiac-specific serology should be obtained as confirmatory testing before starting a gluten-free diet 1
- The test can be used for monitoring, though antibody levels may persist or fluctuate during treatment 4, 9