tTG-IgA — Celiac Disease
Test Characteristics
| Metric | Value |
|---|---|
| False-negative rate | 5% (sensitivity 95%) |
| False-positive rate | 7% (specificity 93%) |
| Bayes factor (positive test) | 10× |
| Bayes factor (negative test) | 1/20× |
| Base rate | 1% of the general population |
Interpreting Results
| Scenario | Prior | + Result | − Result |
|---|---|---|---|
| Unselected patient | 1% | 10 × 1% = 10%1%× 1010% | 1/20 × 1% = 0.05%1%÷ 200.05% |
| Chronic diarrhea, weight loss, iron deficiency | 5% | 10 × 5% = 50%5%× 1050% | 1/20 × 5% = 0.25%5%÷ 200.25% |
| First-degree relative of celiac patient | 10% | 10 × 10% ≥ 100%10%× 10100% | 1/20 × 10% = 0.5%10%÷ 200.5% |
10 × 10% ≥ 100%10%× 10100%: exact posterior is 53%. Highly elevated titers (≥10× ULN) have a much stronger Bayes factor (~100×) and are now accepted for biopsy-free diagnosis. IgA-deficient patients (~2–3%) require IgG-based testing.
- + result: risk goes from shellfish allergy (1%) to left-handed (10%)
- − result: risk drops to near fatal car crash (0.05%)
Sources:
- Systematic review, Gastroenterology, 2024. Gastroenterology.
- Clinical Chemistry study, 2009. PubMed 19799890.
- Beyond Celiac. Prevalence of celiac disease.