What are the steps for testing and diagnosing celiac disease?

Celiac Disease Testing and Diagnosis

Initial Serological Testing

Begin with IgA tissue transglutaminase antibody (tTG-IgA) as the first-line screening test while the patient is consuming a gluten-containing diet, followed by intestinal biopsy for confirmation in adults. 1

Primary Screening Test

• tTG-IgA is the preferred initial test with sensitivity of 90-96% and specificity >95% 1, 2
• Measure total IgA level simultaneously to rule out IgA deficiency, which occurs in 1-3% of celiac patients and causes false-negative results 1, 2
• Testing must be performed while consuming adequate gluten: at least 10g daily (approximately 3 slices of wheat bread) for 6-8 weeks 1, 2

Confirmatory Serological Testing

• If tTG-IgA is >10× upper limit of normal, perform IgA endomysial antibody (EMA-IgA) as confirmatory test with excellent specificity of 99.6% 1, 2
• The combination of tTG-IgA >10× upper limit of normal plus positive EMA-IgA approaches 100% positive predictive value for celiac disease 1

Intestinal Biopsy (Gold Standard)

Upper endoscopy with small bowel biopsy remains the gold standard for diagnosis in adults, even with highly positive serology. 1, 2

Biopsy Technique

• 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 1, 2
• This multi-site sampling increases diagnostic accuracy and accounts for patchy distribution of villous atrophy 1

Histologic Findings

• Characteristic findings include villous atrophy, crypt lengthening, and increased intraepithelial lymphocytes 1, 2
• Marsh classification is used to grade severity of intestinal damage 2

Special Testing Scenarios

IgA Deficiency

• In IgA-deficient patients, use IgG-based tests: IgG deamidated gliadin peptide (DGP-IgG) or IgG tissue transglutaminase (tTG-IgG) 1, 2
• IgG-based tests should NOT be used in patients with normal IgA levels, as they are markedly less accurate in this setting 1

Children Under 2 Years

• Combine tTG-IgA with IgG and IgA deamidated gliadin peptides to improve sensitivity 1, 2

Pediatric Biopsy-Avoidance Protocol

• In children with tTG-IgA ≥10× upper limit of normal, positive EMA-IgA, AND positive HLA-DQ2/DQ8, biopsy may be avoided 1, 2
• This protocol does NOT apply to adults, who should still undergo biopsy confirmation 1

Genetic Testing (HLA-DQ2/DQ8)

HLA testing has >99% negative predictive value—absence of both DQ2 and DQ8 alleles essentially rules out celiac disease. 1

When to Use HLA Testing

• Celiac disease strongly suspected despite negative serology 1
• Equivocal biopsy findings 1
• Patient already on gluten-free diet who was never properly tested 1
• Discrepant serology and histology results 2

Diagnostic Algorithm for Negative Serology with High Clinical Suspicion

1. Confirm adequate gluten intake (at least 10g daily for 6-8 weeks) 1
2. Verify total IgA level is normal (not IgA deficient) 1
3. Perform HLA-DQ2/DQ8 testing—if positive, proceed to biopsy 1
4. If patient already started gluten-free diet, resume normal diet with 3 slices of wheat bread daily for 1-3 months (some patients may require several years for histologic relapse) before repeat testing 1

Critical Pitfalls to Avoid

Never Start Gluten-Free Diet Before Testing

• Initiating gluten-free diet before completing diagnostic workup leads to false-negative results and inconclusive biopsies 1, 2
• This is the most common diagnostic error in celiac disease 1

Do Not Rely on Serology Alone in Adults

• Biopsy confirmation is required in adults to avoid misdiagnosis, even with highly positive serology 1, 2
• Seronegative celiac disease can occur and requires biopsy for diagnosis 1

Avoid IgG Testing in Non-IgA Deficient Patients

• IgG-based tests have poor specificity in patients with normal IgA levels and should not be used 1

Post-Diagnosis Follow-Up Testing

Perform follow-up tTG-IgA at 6 months and 12 months after diagnosis, then yearly thereafter. 1, 2

Interpretation of Follow-Up Serology

• Negative serology after treatment does NOT guarantee intestinal mucosal healing 1, 2
• Persistently positive serology usually indicates ongoing gluten exposure and continued intestinal damage 1, 2
• Serology has low sensitivity (only 50% for tTG-IgA and 45% for EMA-IgA) for detecting persistent villous atrophy in patients on gluten-free diet 3

When to Repeat Biopsy

• Patients with persistent or relapsing symptoms should undergo repeat endoscopic biopsies to determine healing, even with negative tTG-IgA 1
• Consider follow-up biopsy in 1-3 years to confirm mucosal healing, especially in severe initial presentation 4

High-Risk Groups Requiring Screening

• First-degree relatives of celiac patients 1
• Type 1 diabetes mellitus 1
• Unexplained iron deficiency anemia 1
• Autoimmune thyroid disease 1
• Premature osteoporosis 1
• Unexplained liver transaminase elevations 1
• Down syndrome 1