What are the steps for testing and diagnosing celiac disease?

Celiac Disease Testing and Diagnosis

Initial Screening Test

Begin with IgA tissue transglutaminase antibody (tTG-IgA) plus total IgA level measurement while the patient consumes a gluten-containing diet—this is the preferred first-line approach with sensitivity of 90-96% and specificity exceeding 95%. 1, 2, 3

Critical Pre-Testing Requirements

• The patient must be consuming at least 10g of gluten daily for 6-8 weeks before testing—starting a gluten-free diet before diagnostic workup causes false-negative results and invalidates the testing. 1, 2, 3
• If a patient has already started a gluten-free diet, they must resume eating three slices of wheat bread daily for 1-3 months (sometimes requiring several years for histologic relapse) before repeat testing can be performed. 2

Simultaneous Total IgA Measurement

• Measure total IgA level alongside tTG-IgA to identify IgA deficiency, which occurs in 1-3% of celiac patients and causes false-negative tTG-IgA results. 1, 3
• Do not routinely measure IgA levels in all patients as a first step unless IgA deficiency is strongly suspected—it is not warranted universally. 4

Confirmatory Serological Testing

When tTG-IgA is elevated, particularly >10× the upper limit of normal, perform IgA endomysial antibody (EMA-IgA) as confirmatory testing with excellent specificity of 99.6%. 1, 2, 3

• The combination of tTG-IgA >10× upper limit of normal plus positive EMA-IgA approaches 100% positive predictive value for celiac disease. 2, 3

Intestinal Biopsy Confirmation

Upper endoscopy with multiple duodenal biopsies remains the gold standard for diagnosis in adults and must be performed despite high antibody levels. 1, 2, 3

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, 3
• Multiple specimens increase diagnostic accuracy and account for patchy distribution of villous atrophy. 2

Histologic Findings

• Characteristic findings include villous atrophy, crypt lengthening, and increased intraepithelial lymphocytes. 1, 2
• Villous atrophy is defined as Marsh 3 lesion or villous height:crypt depth ratio below 3.0. 5

When Biopsy May Be Avoided

• In children only 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
• Biopsy remains mandatory in adults to establish definitive diagnosis and rule out other causes of villous atrophy. 3

Testing in Special Populations

IgA-Deficient Patients

• Use IgG-based tests: IgG deamidated gliadin peptide (DGP-IgG) or IgG tissue transglutaminase (tTG-IgG). 1, 2, 3
• Do not use IgG-based tests in patients with normal IgA levels—they are markedly less accurate in this setting. 2

Children Under 2 Years

• Combine tTG-IgA with IgG and IgA deamidated gliadin peptides for improved sensitivity. 1, 3

HLA Genetic Testing

HLA-DQ2 and HLA-DQ8 testing has >99% negative predictive value—absence of both alleles essentially rules out celiac disease. 1, 2, 3

When to Use HLA Testing

• When celiac disease is strongly suspected despite negative serology. 1, 2, 3
• In patients with equivocal small-bowel histologic findings. 1, 2
• In patients already on a gluten-free diet without prior testing. 1, 2
• In patients with discrepant celiac-specific serology and histology. 1

When NOT to Use HLA Testing

• HLA testing adds no diagnostic value when tTG-IgA is highly elevated—do not use it for diagnosis confirmation in this scenario. 3

Algorithm for Negative Serology with High Clinical Suspicion

1. Confirm patient is consuming adequate gluten (at least 10g daily for 6-8 weeks). 2
2. Verify total IgA level is normal to exclude IgA deficiency. 2
3. Perform HLA-DQ2/DQ8 testing—if positive, proceed to biopsy. 2
4. If HLA is negative, celiac disease is essentially ruled out. 2

High-Risk Groups Requiring Testing

Test the following populations even without symptoms:

• First-degree relatives of celiac patients 4, 2
• Type 1 diabetes mellitus 4, 2
• Unexplained iron deficiency anemia 4, 2
• Premature osteoporosis 4, 2
• Autoimmune thyroid disease 4, 2
• Down syndrome 4, 2
• Turner syndrome 4, 3
• Unexplained liver transaminase elevations 4, 2, 3
• Primary biliary cirrhosis 4
• Autoimmune hepatitis 4

Selective Testing Situations

Consider testing when symptoms compatible with celiac disease are present in:

• Sjögren's syndrome 4
• Unexplained recurrent fetal loss 4
• Unexplained delayed puberty 4
• Selective IgA deficiency 4
• Irritable bowel syndrome 4
• Peripheral neuropathy 4
• Cerebellar ataxia 4
• Recurrent migraine 4
• Children with short stature 4

Critical Pitfalls to Avoid

• Never initiate a gluten-free diet before completing diagnostic testing—this leads to false-negative results and invalidates the workup. 1, 2
• Never rely solely on serology without biopsy confirmation in adults—this leads to misdiagnosis. 1, 2
• Do not use antigliadin antibody (AGA) testing—it has poor diagnostic performance compared to tTG-IgA and EMA-IgA. 4, 6
• Lymphocytic infiltration without villous atrophy is not specific for celiac disease and requires consideration of other causes. 1
• Symptom improvement on a gluten-free diet has low positive predictive value and should not be used for diagnosis without supportive evidence. 1
• Seronegative celiac disease can occur and requires biopsy for diagnosis. 2

Follow-Up Testing After Diagnosis

• Perform follow-up serology at 6 and 12 months after diagnosis, then yearly thereafter. 1, 2
• Negative serology after treatment does not guarantee intestinal mucosal healing—antibody tests have only 50% sensitivity for detecting persistent villous atrophy in patients on a gluten-free diet. 2, 5
• Persistently elevated antibodies usually indicate ongoing gluten exposure and continued intestinal damage. 1, 2
• Patients with persistent or relapsing symptoms require repeat endoscopic biopsies to determine healing, even with negative tTG-IgA. 2