What is the recommended serum testing algorithm for diagnosing celiac disease in a patient who is currently consuming a gluten‑containing diet?

Serum Testing for Celiac Disease

The recommended initial serum test is IgA tissue transglutaminase antibody (tTG-IgA) measured simultaneously with total IgA level, and the patient must be consuming at least 10 grams of gluten daily for 6–8 weeks before testing. 1, 2, 3

Step 1: Initial Screening Panel

• Order tTG-IgA plus total IgA as your first-line screening approach—this combination is the most efficient initial strategy endorsed by major gastroenterology societies. 1, 3

• The tTG-IgA test demonstrates strong diagnostic performance: in adults, sensitivity reaches approximately 90.7% and specificity 87.4% at a 15 U/mL threshold; in children, sensitivity climbs to 97.7% with specificity of 70.2% at a 20 U/mL cutoff. 1, 3

• Measuring total IgA is mandatory because selective IgA deficiency occurs in 1–3% of celiac patients and causes falsely negative IgA-based antibody results, potentially missing the diagnosis entirely. 1, 2, 3

Step 2: Confirmatory Testing When tTG-IgA Is Positive

• Add IgA endomysial antibody (EMA-IgA) testing when tTG-IgA is elevated, because EMA provides superior specificity of approximately 99.6% in adults and 93.8% in children. 1, 3

• When tTG-IgA exceeds 10 times the upper limit of normal and EMA-IgA is positive on a second blood sample, the positive predictive value approaches 100% for celiac disease. 1, 2

• This two-step approach (tTG first, then EMA confirmation) yields a sensitivity of 85.7%, specificity of 98.6%, positive predictive value of 71.7%, and negative predictive value of 99.7%. 4

Step 3: Alternative Testing for IgA-Deficient Patients

• If total IgA is low or absent, switch to IgG-based serology rather than relying on IgA tests. 1, 3

• IgG deamidated gliadin peptide (DGP-IgG) is the preferred IgG test, demonstrating excellent accuracy with sensitivity of 93.6% and specificity of 99.4% in adults. 1, 3

• IgG tTG has inferior performance (sensitivity only 40.6–84.6%, specificity 78.0–89.0%) and should not be used when DGP-IgG is available. 1

• IgG EMA shows near-universal positivity in IgA-deficient celiac patients in research cohorts. 3

• Never use IgG-based tests for primary screening when total IgA is normal—this generates frequent false-positive results and reduces diagnostic accuracy. 1, 3

Critical Pre-Testing Requirements

• Patients must consume at least 10 grams of gluten daily for 6–8 weeks before any serologic testing to avoid false-negative results; insufficient gluten exposure is the leading cause of missed diagnoses. 1, 2, 3

• If a patient has already reduced or eliminated gluten, they must resume normal gluten intake (equivalent to approximately three slices of wheat bread daily) for 1–3 months before repeat testing. 1, 3

• Never initiate a gluten-free diet before completing the full diagnostic workup—doing so invalidates both serology and subsequent intestinal biopsy findings. 1, 2, 3

Special Population Considerations

• In children younger than 2 years, combine tTG-IgA with deamidated gliadin peptide IgG/IgA testing to improve sensitivity in this age group. 2, 3

• In high-risk populations (first-degree relatives of celiac patients, type 1 diabetes, autoimmune thyroid disease, unexplained iron-deficiency anemia), proceed to biopsy even with borderline or weakly positive serology because pre-test probability is substantially elevated. 1

Common Diagnostic Pitfalls to Avoid

• Do not rely on serology alone to diagnose celiac disease in adults—upper endoscopy with at least six duodenal biopsies (1–2 from the bulb, ≥4 from the second portion of the duodenum or beyond) remains mandatory for confirmation. 2, 3

• Do not order IgG tests when total IgA is normal—this creates confusion with false-positive results and has no role in routine screening. 1, 3

• Do not use older non-deamidated IgA/IgG gliadin antibody assays—these provide no additional diagnostic benefit and have been superseded by more accurate tests. 1, 4

• Do not skip total IgA measurement—approximately 1 in 50 celiac patients has IgA deficiency, and missing this will lead to a false-negative tTG-IgA result. 1, 2, 3

Role of HLA Genetic Testing

• HLA-DQ2/DQ8 typing is not a routine screening tool but is reserved for specific scenarios: equivocal intestinal histology, patients already on a gluten-free diet without prior testing, discordant serology and histology, or when celiac disease is strongly suspected despite negative serology. 2, 3

• Absence of both HLA-DQ2 and HLA-DQ8 alleles provides >99% negative predictive value and essentially excludes celiac disease. 2, 3

• Because HLA-DQ2 is present in 25–30% of the general White population, a positive result does not confirm disease—it only indicates genetic susceptibility. 2

Monitoring After Diagnosis

• Follow-up tTG-IgA testing should occur at 6 months after starting a gluten-free diet, again at 12 months, and then annually thereafter to monitor dietary adherence and mucosal healing. 1

• Persistently positive serology indicates ongoing intestinal damage from continued gluten exposure. 1

• A normal tTG level does not guarantee recovery of villous atrophy—44% of patients with persistent villous atrophy on biopsy had normal tTG levels in one study. 4