What is the recommended lab test for diagnosing gluten sensitivity?

Lab Test for Gluten Sensitivity (Celiac Disease)

The IgA tissue transglutaminase antibody (tTG-IgA) is the recommended first-line lab test for diagnosing celiac disease, and should be performed alongside a total IgA level to rule out IgA deficiency. 1, 2, 3

Initial Serologic Testing Approach

Primary Test

• IgA tissue transglutaminase (tTG-IgA) is the most efficient single serologic test with sensitivity of 90-96% and specificity >95% 1, 2, 3
• Must be performed while the patient is consuming a gluten-containing diet (at least 10g daily for 6-8 weeks) to avoid false-negative results 2, 3
• Testing after gluten withdrawal leads to resolution of positive serologic results and inconclusive findings 1, 3

Concurrent Testing

• Total IgA level must be measured simultaneously with tTG-IgA to identify IgA deficiency, which occurs in 1-3% of celiac disease patients and causes falsely low results 1, 4, 2, 3
• The American Gastroenterological Association recommends this two-test strategy as standard practice 1

Confirmatory and Additional Testing

When tTG-IgA is Positive

• IgA endomysial antibody (EMA-IgA) should be performed as a confirmatory test when tTG-IgA is weakly positive or when levels are >10× the upper limit of normal 1, 2
• EMA has slightly lower sensitivity but excellent specificity of 99.6% 1, 2
• The concordance rate between tTG-IgA and EMA-IgA is approximately 95-100% in untreated celiac disease 5, 6

For IgA-Deficient Patients

• Use IgG-based tests instead: IgG tissue transglutaminase (tTG-IgG) or IgG deamidated gliadin peptide (DGP-IgG) 1, 4, 2
• IgG DGP has sensitivity of 93.6% and specificity of 99.4% in adults 4
• IgG-based tests are markedly less sensitive and specific than IgA-based tests in patients with normal IgA levels 1

Special Populations

• Children under 2 years: Combine tTG-IgA with IgG and IgA deamidated gliadin peptides for improved sensitivity 2, 3
• Anti-tTG seronegative patients with enteropathy: Consider testing for antibodies to transglutaminase 3 (TG3) or transglutaminase 6 (TG6), which can detect an additional 36.8% of cases 7

Tests to Avoid

• IgA antigliadin antibodies (AGA) are not recommended as the diagnostic performance is inferior to tTG-IgA and EMA 1
• The American Gastroenterological Association explicitly states that additional inclusion of IgG and IgA antigliadin antibodies is not warranted in primary care 1

Gold Standard Confirmation

• Upper endoscopy with small bowel biopsy remains the gold standard for establishing diagnosis in adults 1, 4, 2, 3
• Multiple duodenal biopsies are required: ideally 6 specimens from the second part of the duodenum or beyond, including 1-2 from the duodenal bulb 1, 2, 3
• Characteristic histologic findings include villous atrophy (partial to total), crypt lengthening, increased lamina propria, and increased intraepithelial lymphocytes 1, 2, 3

Biopsy-Avoidance Strategy (Pediatrics)

• In children with tTG-IgA ≥10× upper limit of normal, positive EMA-IgA, and positive HLA-DQ2/DQ8, biopsy may be avoided 1, 2, 3
• This approach was adopted during the COVID-19 pandemic for adults but is not standard practice outside that context 1

HLA Genetic Testing

• HLA-DQ2 and HLA-DQ8 testing has high negative predictive value (>99%) but limited positive predictive value 2, 3
• Useful when celiac disease is strongly suspected despite negative serology, or in patients with equivocal histologic findings 4, 2, 3
• Absence of both HLA-DQ2 and HLA-DQ8 makes celiac disease highly unlikely 2, 3

Critical Pitfalls to Avoid

• Never initiate a gluten-free diet before completing diagnostic testing, as this leads to false-negative serology and inconclusive biopsies 4, 2, 3
• Do not rely solely on serology without biopsy confirmation in adults, as this can lead to misdiagnosis especially with discordant results 4, 2
• Do not measure serum IgA routinely as a first step unless IgA deficiency is strongly suspected; it is an appropriate next step only if tTG-IgA is negative but clinical suspicion remains high 1
• Improvement of symptoms on a gluten-free diet has low positive predictive value and should not be used for diagnosis 2

High-Risk Groups Warranting Testing

Testing should be considered in patients with:

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

Monitoring After Diagnosis

• Follow-up tTG-IgA testing should be performed at 6 months, 12 months, then annually after starting a gluten-free diet 4, 2
• Persistently elevated antibodies indicate ongoing gluten exposure or poor dietary compliance 4, 2
• Important caveat: Serology has low sensitivity (approximately 50%) for detecting persistent villous atrophy in patients on a gluten-free diet, so negative serology does not guarantee mucosal healing 8
• A decreasing trend in tTG-IgA levels may be a better marker of compliance than absolute normalization 9