In a patient being evaluated for celiac disease because of gluten‑related symptoms, weight loss, anemia, iron deficiency, or a family history, how should I order and interpret tissue transglutaminase immunoglobulin A (tTG‑IgA) testing, including the need for total serum IgA, confirmatory endomysial antibody testing, biopsy indications, and use of IgG‑based assays if IgA deficiency is present?

Tissue Transglutaminase IgA Testing for Celiac Disease

Ordering the Test

Order tissue transglutaminase IgA (tTG-IgA) together with total serum IgA as the first-line screening test for celiac disease in any patient consuming gluten who presents with suggestive symptoms, unexplained iron deficiency, weight loss, or a family history of celiac disease. 1

Pre-Test Requirements

• Ensure the patient has consumed at least 10 grams of gluten daily (approximately 3 slices of wheat bread) for 6–8 weeks before testing, as insufficient gluten exposure is the leading cause of false-negative results. 1, 2
• Never initiate a gluten-free diet before completing the full diagnostic workup, as this invalidates both serologic and histologic testing. 1

Why Measure Total IgA Simultaneously

• IgA deficiency occurs in approximately 1–3% of celiac disease patients (10–15 times higher than the general population) and causes falsely negative IgA-based antibody tests. 1, 3
• Measuring total IgA identifies patients who require IgG-based testing instead of standard IgA assays. 1
• A tTG-IgA value ≥0.10 μ/mL effectively excludes IgA deficiency with 92% sensitivity and 84% specificity, meaning only 16% of patients with values below this threshold require total IgA measurement. 4

Interpreting Results

Positive tTG-IgA with Normal Total IgA

• When tTG-IgA is positive, order endomysial antibody (EMA) testing as confirmatory serology because EMA has superior specificity of approximately 99.6% in adults. 1, 2
• If tTG-IgA exceeds 10 times the upper limit of normal and a second blood sample is EMA-positive, the positive predictive value for celiac disease approaches 100%. 1
• The concordance rate between tTG-IgA and EMA is approximately 95%, but tTG-IgA may detect 7.6% of celiac patients who are EMA-negative. 5, 6

Negative tTG-IgA with Normal Total IgA

• A negative tTG-IgA with normal total IgA effectively excludes celiac disease in most patients (sensitivity 90.7% in adults, 97.7% in children). 1, 2
• If clinical suspicion remains high despite negative serology, verify adequate gluten intake and proceed directly to upper endoscopy with at least 6 duodenal biopsies to assess for seronegative celiac disease. 1, 2
• Consider HLA-DQ2/DQ8 genetic testing in equivocal cases; absence of both alleles has >99% negative predictive value and essentially excludes celiac disease. 1, 2

IgA Deficiency Detected (Low or Absent Total IgA)

• Immediately order IgG deamidated gliadin peptide (DGP-IgG) as the preferred test in IgA-deficient patients, with diagnostic accuracy of 93.6% sensitivity and 99.4% specificity. 1, 3
• Do not rely on IgG tTG, as it has poor diagnostic accuracy (sensitivity only 40.6–84.6%, specificity 78.0–89.0%) and should not be used to exclude celiac disease. 2, 3
• A borderline or low tTG-IgA result (e.g., 1.0 U/mL) in the setting of low total IgA is often falsely negative and should prompt IgG-based testing. 3

Biopsy Indications

Proceed to Endoscopy When:

• tTG-IgA is positive (≥15 U/mL in adults, ≥20 U/mL in children), regardless of EMA status. 1, 2
• tTG-IgA exceeds 10 times the upper limit of normal combined with positive EMA in a second sample (virtually 100% positive predictive value). 1
• Clinical suspicion remains high despite negative serology, especially in high-risk populations (first-degree relatives, type 1 diabetes, autoimmune thyroid disease, unexplained iron deficiency). 1, 2
• IgG DGP is positive in an IgA-deficient patient. 3

Biopsy Protocol

• Obtain at least 6 duodenal biopsy specimens: 1–2 from the duodenal bulb and ≥4 from the second portion of the duodenum or beyond, as mucosal changes can be patchy. 1, 2
• Request evaluation by a pathologist with gastroenterology expertise using Marsh classification, as poorly oriented specimens lead to misinterpretation. 1, 2

Common Pitfalls and How to Avoid Them

Testing Errors

• Never order multiple antibody panels simultaneously in low-risk populations, as this reduces specificity without meaningful gains in sensitivity. 1, 2
• Do not use IgG tTG as a primary screening test when total IgA is normal, as it lacks specificity and generates frequent false-positives. 1, 2
• Avoid relying on outdated IgA/IgG gliadin antibody assays (non-deamidated), as they provide no additional diagnostic benefit. 2

Clinical Management Errors

• Never diagnose celiac disease based solely on serology without biopsy confirmation in adults, even with strongly positive antibodies. 1
• Do not start a gluten-free diet before completing diagnostic evaluation, as this invalidates subsequent testing and prevents definitive diagnosis. 1, 2
• Recognize that approximately 6–7% of celiac disease cases are antibody-negative (seronegative celiac disease), requiring biopsy despite negative serology when clinical suspicion is high. 3, 7

Interpretation Errors

• An isolated positive IgG tTG with negative IgA tTG and normal total IgA has limited diagnostic value and should not prompt biopsy unless the patient is high-risk. 2
• A weakly positive deamidated gliadin peptide (DGP) result requires confirmation with tTG-IgA and total IgA before proceeding to biopsy. 2
• Elevated total IgA levels (unrelated to celiac disease) actually strengthen confidence in negative tTG-IgA results, as they confirm the IgA-based test is valid. 2

Special Populations

High-Risk Groups Requiring Lower Threshold for Biopsy

• First-degree relatives of celiac patients (7.5% prevalence). 2
• Type 1 diabetes (5–10% prevalence). 2
• Autoimmune thyroid disease or autoimmune liver disease. 2
• Unexplained iron deficiency anemia (2–6% prevalence). 1, 2
• Down syndrome or Turner syndrome. 2

Children Under 2 Years

• Combine tTG-IgA testing with IgG and IgA deamidated gliadin peptides to improve sensitivity in this age group. 1

Monitoring After Diagnosis

• Repeat tTG-IgA (or IgG-based tests in IgA-deficient patients) at 6 months, 12 months, and annually thereafter to monitor dietary adherence. 2, 3
• Persistently positive serology indicates ongoing intestinal damage and gluten exposure. 2
• Consider repeat duodenal biopsy 1–3 years after diagnosis in patients with severe initial disease to confirm mucosal healing. 2, 3