What is the recommended approach for testing and diagnosing celiac disease in patients with suspected symptoms or a family history of the condition?

Testing for Celiac Disease

Begin with IgA tissue transglutaminase antibody (tTG-IgA) testing while the patient is on a gluten-containing diet, measure total IgA levels simultaneously to rule out IgA deficiency, and proceed to upper endoscopy with multiple duodenal biopsies for histologic confirmation in adults. 1

Initial Serologic Testing

The diagnostic workup must start with the right serologic test while ensuring the patient is consuming adequate gluten:

• tTG-IgA is the preferred first-line screening test with sensitivity of 90-96% and specificity >95% 2, 1
• Total IgA levels must be measured simultaneously to identify IgA deficiency, which occurs in 1-3% of celiac disease patients and causes falsely negative IgA-based tests 2, 1
• Testing requires adequate gluten intake: at least 10g daily (approximately 3 slices of wheat bread) for 6-8 weeks prior to testing 2, 1
• Never initiate a gluten-free diet before completing diagnostic testing, as this leads to false-negative serology and inconclusive biopsies 2, 1

Confirmatory Serologic Testing

When tTG-IgA is elevated, additional testing strengthens diagnostic certainty:

• For tTG-IgA levels >10× upper limit of normal, obtain IgA endomysial antibody (EMA-IgA) as confirmatory testing with specificity of 99.6% 2, 1
• The combination of strongly positive tTG-IgA (>10× upper limit) plus positive EMA-IgA has virtually 100% positive predictive value for celiac disease 2, 1
• In adults with this serologic pattern, biopsy may still be performed primarily for differential diagnosis purposes 2

Testing in IgA-Deficient Patients

When total IgA is below the lower limit of detection, switch to IgG-based testing:

• Use IgG deamidated gliadin peptide (DGP-IgG) as the preferred test with superior diagnostic accuracy (93.6% sensitivity, 99.4% specificity) compared to IgG tTG 2, 1
• IgG tTG testing is not specific in the absence of IgA deficiency and should be avoided 2
• Patients with IgA deficiency and positive IgG-based tests should be considered to have celiac disease with selective IgA deficiency, not seronegative celiac disease 2

Histologic Confirmation

Upper endoscopy with duodenal biopsies remains the gold standard in adults:

• 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 2, 1
• Multiple biopsies are essential due to the patchy distribution of celiac disease 1
• Request evaluation by a pathologist with gastroenterology expertise using Marsh classification, counting intraepithelial lymphocytes per high-power field, and morphometry 2
• Characteristic findings include villous atrophy, crypt hyperplasia, and increased intraepithelial lymphocytes 1

Genetic Testing: Limited but Strategic Role

HLA-DQ2/DQ8 testing should be used selectively, not routinely:

• HLA testing has >99% negative predictive value—absence of both DQ2 and DQ8 essentially rules out celiac disease 2, 1
• Use HLA testing when celiac disease is strongly suspected despite negative serology, in patients with equivocal biopsy findings, or in patients already on a gluten-free diet who were never properly tested 2, 1
• Do not use HLA testing as a primary diagnostic tool; its value lies in ruling out disease, not confirming it 2

Special Diagnostic Scenarios

Seronegative Celiac Disease

When clinical suspicion remains high despite negative serology:

• Confirm the patient was consuming adequate gluten (≥10g daily for 6-8 weeks) when tested 2
• Verify total IgA level was measured to exclude IgA deficiency 2
• Proceed to upper endoscopy with duodenal biopsies if clinical suspicion remains high 2
• If biopsies show villous atrophy with negative serology and positive HLA-DQ2/DQ8, confirm diagnosis by demonstrating clinical and histologic improvement on a gluten-free diet with repeat endoscopy in 1-3 years 2

Patients Already on Gluten-Free Diet

When patients have started a gluten-free diet before diagnostic testing:

• Resume normal diet with three slices of wheat bread daily for 1-3 months before repeat serologic testing and biopsy 2, 1
• Some patients may require several years of gluten challenge for histologic relapse to occur 1
• This is a common pitfall that significantly complicates diagnosis 2

Pediatric Biopsy-Avoidance Protocol

In children, biopsy may be avoided under specific conditions:

• Children with tTG-IgA ≥10× upper limit of normal, positive EMA-IgA, and positive HLA-DQ2/DQ8 may be diagnosed without biopsy according to pediatric guidelines 1
• In children under 2 years, combine tTG-IgA with IgG and IgA deamidated gliadin peptides to improve sensitivity 1, 3

High-Risk Groups Requiring Testing

Screen the following populations even without typical symptoms:

• First-degree relatives of patients with celiac disease 1
• Patients with type 1 diabetes mellitus 1
• Patients with unexplained iron deficiency anemia 1
• Patients with autoimmune thyroid disease 1
• Patients with Down syndrome 1
• Patients with premature osteoporosis 1
• Patients with unexplained liver transaminase elevations 1

Common Diagnostic Pitfalls

Several errors frequently compromise diagnostic accuracy:

• Starting a gluten-free diet before testing is the most common pitfall, leading to false-negative results 2, 1
• Relying solely on serology without biopsy confirmation in adults can lead to misdiagnosis, especially with discordant results 2, 3
• Failing to measure total IgA levels alongside tTG-IgA misses IgA deficiency 2
• Using IgG-based tests in patients with normal IgA levels reduces specificity 2
• Inadequate biopsy sampling (fewer than 6 specimens) or poorly oriented specimens lead to false-negative histology 2, 1
• Confusing symptom improvement on a gluten-free diet with diagnostic confirmation—this has low positive predictive value and cannot differentiate celiac disease from non-celiac gluten sensitivity 3

Post-Diagnosis Monitoring

After confirming celiac disease and initiating treatment:

• Refer to a registered dietitian experienced in celiac disease management for comprehensive dietary education 1
• Perform follow-up tTG-IgA testing at 6 months, 12 months, and annually thereafter 2, 1
• Persistently positive serology indicates ongoing intestinal damage and continued gluten exposure 2, 1
• Negative serology does not guarantee mucosal healing—serologic tests have only 50% sensitivity for detecting persistent villous atrophy in patients on a gluten-free diet 4
• Patients with persistent or relapsing symptoms should undergo repeat endoscopic biopsies to determine healing status, even with negative serology 2, 1