What are the diagnostic tests for gluten and dairy intolerance?

Diagnostic Tests for Gluten and Dairy Intolerance

For accurate diagnosis of gluten and dairy intolerance, serological testing and intestinal biopsy are the gold standards for celiac disease, while lactose breath hydrogen testing is recommended for lactose intolerance.

Gluten-Related Disorders Diagnostic Tests

Celiac Disease Testing

Celiac disease requires specific diagnostic testing while the patient is on a gluten-containing diet:

1. First-line testing:

   • IgA tissue transglutaminase antibody (TG2-IgA) - Primary screening test for those over 2 years old 1
   • Total IgA levels - To rule out IgA deficiency which could cause false negatives 1

2. Additional serological tests:

   • IgA endomysial antibody (EMA) - Highly specific confirmatory test 1
   • IgG deamidated gliadin peptide (DGP) - For IgA-deficient patients 1
   • IgG tissue transglutaminase - For IgA-deficient patients 1

3. Confirmatory testing:

   • Upper endoscopy with multiple duodenal biopsies - Gold standard for diagnosis 1
     • At least 4-6 biopsies from distal duodenum
     • 1-2 biopsies from duodenal bulb

4. Genetic testing:

   • HLA-DQ2/DQ8 typing - Not for routine diagnosis but useful in:
     • Patients already on gluten-free diet without prior testing
     • Discrepant serology and histology results
     • Equivocal biopsy findings 1

Non-Celiac Gluten Sensitivity Testing

For non-celiac gluten sensitivity, diagnosis is made after excluding celiac disease and wheat allergy:

1. Serological pattern:

   • Positive IgG anti-gliadin antibodies (AGA) in ~56% of cases
   • Negative tissue transglutaminase antibodies
   • Negative endomysial antibodies
   • Negative deamidated gliadin peptide antibodies 2

2. Diagnostic approach:

   • Rule out celiac disease with negative celiac-specific antibodies
   • Normal duodenal histology or mild abnormalities
   • Symptom improvement on gluten-free diet
   • Symptom return with gluten challenge 1

Dairy Intolerance Testing

Lactose Intolerance Testing

1. Lactose breath hydrogen test:

   • Non-invasive gold standard test
   • Measures hydrogen in breath after lactose consumption
   • Positive result indicates lactose malabsorption 3

2. Other testing options:

   • Lactose tolerance test (blood glucose measurement)
   • Stool acidity test (primarily in infants)

Important Clinical Considerations

Testing Pitfalls to Avoid

1. For celiac disease:

   • Do not start gluten-free diet before testing - This reduces sensitivity of both serology and biopsy 1
   • If already on gluten-free diet, gluten challenge (3 slices of wheat bread daily for 1-3 months) is needed before testing 1
   • Antibody tests against native gliadin are no longer recommended 1
   • Stool studies, small-bowel follow-through, intestinal permeability testing, D-xylose testing, and salivary testing are not recommended 1

2. For non-celiac gluten sensitivity:

   • Diagnosis should only be considered after celiac disease has been ruled out 1
   • Symptom response to gluten-free diet alone is insufficient for diagnosis 1

3. For lactose intolerance:

   • Symptoms alone cannot reliably diagnose lactose intolerance
   • Objective testing is required to confirm malabsorption 3

Monitoring After Diagnosis

For celiac disease:

• Follow-up serology at 6 and 12 months after diagnosis, then yearly 1
• Negative serology doesn't guarantee intestinal healing 1
• Persistently positive serology usually indicates ongoing gluten exposure 1
• Consider repeat biopsy for persistent or relapsing symptoms 1

Diagnostic Algorithm

1. For suspected gluten-related disorder:

   • Start with TG2-IgA and total IgA while patient is on gluten-containing diet
   • If positive TG2-IgA → proceed to endoscopy with duodenal biopsies
   • If negative TG2-IgA but high clinical suspicion → consider EMA, DGP, and/or endoscopy
   • If normal histology but symptoms persist → consider non-celiac gluten sensitivity

2. For suspected lactose intolerance:

   • Perform lactose breath hydrogen test
   • Positive test confirms lactose malabsorption
   • Treatment based on dietary lactose reduction

By following these evidence-based diagnostic approaches, clinicians can accurately diagnose gluten and dairy intolerances, leading to appropriate dietary management and improved patient outcomes.