In suspected coeliac disease, which investigation should be performed first: serological antibody testing (anti‑tissue transglutaminase IgA with total IgA) or endoscopic duodenal biopsy?

Initial Investigation for Suspected Coeliac Disease

Serological antibody testing with IgA tissue transglutaminase (tTG-IgA) plus total IgA level should always be performed first in suspected coeliac disease, followed by endoscopic duodenal biopsy for confirmation in adults. 1, 2, 3

First-Line Serological Testing Strategy

• Measure IgA tTG-IgA simultaneously with total IgA level as the initial diagnostic test in all patients with suspected coeliac disease who are consuming gluten. 1, 2, 3

• This combination serves as the most efficient screening approach because total IgA measurement identifies the 1–3% of coeliac patients with IgA deficiency who would otherwise have falsely negative IgA-based antibody results. 1, 2, 3

• IgA tTG-IgA demonstrates strong diagnostic performance: 90.7% sensitivity and 87.4% specificity in adults at 15 U/mL threshold, and 97.7% sensitivity with 70.2% specificity in children at 20 U/mL threshold. 1, 4

Critical Pre-Test Requirements

• Patients must consume at least 10g of gluten daily for 6–8 weeks before testing to avoid false-negative results. 1, 4, 2, 3

• Starting a gluten-free diet before completing diagnostic workup leads to false-negative serology and inconclusive biopsies, creating diagnostic uncertainty. 1, 2, 3

Confirmatory Serological Testing

• When IgA tTG-IgA is positive (especially >10× upper limit of normal), perform IgA endomysial antibody (EMA) testing as a second-line confirmatory test with superior specificity of 99.6% in adults. 1, 4, 2, 3

• The combination of tTG-IgA >10× upper limit of normal plus positive EMA approaches 100% positive predictive value for coeliac disease. 4, 3

Mandatory Biopsy Confirmation in Adults

• Upper endoscopy with duodenal biopsy remains mandatory in adults despite high antibody levels, as it establishes the diagnosis definitively and rules out other causes of villous atrophy. 1, 2, 3

• Obtain at least 6 biopsy specimens total: 1–2 from the duodenal bulb and at least 4 from the second part of the duodenum or beyond, because mucosal changes can be patchy. 4, 2, 3

• Relying solely on serology without biopsy confirmation can lead to misdiagnosis in adults. 1, 2

Special Populations Requiring Modified Testing

• In IgA-deficient patients (total IgA below lower limit of detection), use IgG-based tests instead: IgG deamidated gliadin peptide (DGP-IgG) is preferred with 93.6% sensitivity and 99.4% specificity in adults. 1, 4, 2

• In children under 2 years, combine tTG-IgA with IgG and IgA deamidated gliadin peptides for improved sensitivity. 2, 3

Why Serology Must Come Before Biopsy

• Serological testing is non-invasive, widely available, and highly accurate for identifying patients who require the invasive endoscopic procedure. 1

• Performing biopsy first without serological screening would subject many patients without coeliac disease to unnecessary invasive procedures with associated discomfort and risk. 1, 5

• The positive predictive value of serology increases dramatically at higher antibody thresholds, allowing risk stratification before proceeding to biopsy. 1, 6

Emerging Biopsy-Avoidance Strategies (Context-Specific)

• In children with tTG-IgA ≥10× upper limit of normal, positive EMA-IgA, and positive HLA-DQ2/DQ8, biopsy may be avoided according to pediatric guidelines. 1, 2

• Some evidence suggests adults with very high tTG-IgA levels (>8–10× upper limit of normal) and positive EMA may have 100% positive predictive value for coeliac disease, though this approach is not yet widely accepted in adult guidelines. 7, 6

• However, current best practice in adults still requires biopsy confirmation regardless of antibody levels to establish definitive diagnosis and exclude alternative pathology. 1, 2, 3

Common Diagnostic Pitfalls

• Never diagnose coeliac disease based on symptoms alone or symptom response to gluten-free diet, as this cannot differentiate coeliac disease from non-coeliac gluten sensitivity. 2

• Do not proceed directly to biopsy without serological testing first, as negative serology in a patient consuming adequate gluten effectively rules out coeliac disease in most cases. 1

• Ensure proper specimen orientation during biopsy and request evaluation by a pathologist with gastroenterology expertise, as poorly oriented mucosa leads to misinterpretation. 4