What is the role of a blood test, such as RAST (RadioAllergoSorbent Test) or ELISA (Enzyme-Linked Immunosorbent Assay), in diagnosing dairy intolerance?

Blood Tests for Dairy Intolerance: Limited Utility in Diagnosis

Blood tests such as RAST or ELISA have limited value in diagnosing dairy intolerance and should not be used as standalone diagnostic tools for this condition. 1

Understanding Dairy Intolerance vs. Dairy Allergy

Dairy-related adverse reactions fall into two main categories:

1. Lactose Intolerance:

   • A physiological condition caused by lactase deficiency
   • Symptoms include abdominal pain, bloating, flatulence, and diarrhea occurring 30 minutes to 2 hours after dairy consumption
   • NOT an immune-mediated reaction 2

2. Milk Protein Allergy:

   • True immune-mediated reaction to milk proteins
   • Can be IgE-mediated (immediate) or non-IgE-mediated (delayed) 1, 2

Diagnostic Approach for Dairy Intolerance

For Lactose Intolerance:

1. Hydrogen Breath Test:

   • Most widely used diagnostic test
   • Involves ingestion of 25-50g lactose after overnight fast
   • Rise in hydrogen >20 parts per million suggests lactose malabsorption
   • Note: Has a 25% false negative rate 2

2. Elimination and Challenge:

   • Remove dairy products for 2 weeks
   • Monitor symptom improvement
   • Reintroduce dairy and observe for symptom recurrence 2

3. Symptom Diary:

   • Track symptoms in relation to dairy consumption 2

For Milk Protein Allergy:

1. Skin Prick Testing (SPT):

   • Tests for immediate/Type I hypersensitivity reactions
   • High negative predictive value (>95%)
   • Low specificity and positive predictive value (40-60%) 1

2. Serum-Specific IgE Tests (including RAST and ELISA):

   • Can identify potential IgE-mediated reactions
   • High negative predictive value but low specificity
   • Cannot stand alone as diagnostic tools 1
   • May be preferred when extensive eczema, dermatographism, or recent antihistamine use prevents skin testing 1

3. Oral Food Challenge:

   • Double-blind placebo-controlled food challenge is the gold standard
   • Essential to confirm positive blood or skin test results 1, 3

Limitations of Blood Tests for Dairy Intolerance

1. For Lactose Intolerance:

   • Blood tests measuring IgG antibodies are NOT recommended for diagnosis 2
   • Lactose intolerance is not an immune-mediated condition, so antibody tests are irrelevant

2. For Milk Protein Allergy:

   • Positive blood tests only indicate sensitization, not necessarily clinical allergy
   • Many patients with positive IgE tests fully tolerate dairy products 3
   • The NIAID Food Allergy Expert Panel notes that 50-90% of presumed food allergies are not true allergies when properly tested 1

Key Pitfalls to Avoid

1. Relying solely on blood tests: A positive RAST or ELISA test without clinical correlation can lead to unnecessary dietary restrictions 1, 3

2. Confusing intolerance with allergy: These are distinct conditions requiring different diagnostic approaches 2

3. Using IgG antibody tests: Food elimination diets based on IgG antibodies are not recommended for gastrointestinal disorders including lactose intolerance 2

4. Skipping the gold standard: Oral food challenges remain essential to confirm suspected milk protein allergy 1, 3

Conclusion

While blood tests like RAST and ELISA can help identify potential IgE-mediated milk protein allergies, they have significant limitations and cannot diagnose lactose intolerance. The diagnosis of dairy-related disorders requires a comprehensive approach including clinical history, elimination diets, and appropriate challenge testing.