Most Appropriate Investigation: Specific IgE Testing for Common Food Allergens
For a child under 5 years with suspected atopic dermatitis unresponsive to topical corticosteroids, the most appropriate investigation is specific IgE testing (or skin prick testing) for common food allergens—specifically milk, eggs, wheat, soy, and peanuts. 1
Algorithmic Approach to Investigation
Step 1: Confirm Optimal Topical Treatment First
Before attributing symptoms to food allergy, you must ensure the child has received:
- Appropriate-potency topical corticosteroids for the severity of disease 1
- Adequate emollient use 1
- Critical pitfall: Many children have inadequately treated atopic dermatitis, not food allergy 1
Step 2: Determine If Testing Is Indicated
Testing should only proceed if the child meets these criteria:
- Age under 5 years 1, 2
- Moderate-to-severe atopic dermatitis 1, 2
- Disease persists despite optimized topical management 1, 2
- OR there is a reliable history of immediate reaction after ingesting a specific food 1, 2
Step 3: Choose the Appropriate Test
Answer A (IgE levels for common food allergies) is correct because:
- Specific IgE testing for milk, eggs, wheat, soy, and peanuts is recommended by the American Academy of Dermatology 1
- These foods account for >90% of food allergies in children with atopic dermatitis 3
- Skin prick testing is the preferred first-line test, but specific IgE testing is an appropriate alternative, especially when widespread skin lesions or marked dermatographism are present 1, 2
Answer B (PPD skin test) is incorrect because:
- PPD tests for tuberculosis exposure and has no role in food allergy diagnosis
- This is completely unrelated to the clinical scenario
Answer C (Continue food with supervision) is incorrect because:
- You cannot diagnose food allergy through continued exposure without prior testing 1
- Supervised oral food challenge is the gold standard for confirming diagnosis, but only after initial testing shows sensitization 1, 2
- The proper sequence is: testing → elimination diet (if positive) → supervised oral food challenge for confirmation 1, 2
Understanding Test Interpretation
Important Limitations of IgE Testing
- Positive predictive value is only 40-60% 1
- Positive tests indicate sensitization, not necessarily true clinical allergy 1, 2
- Negative predictive value is >95% 1
- Up to 37% of children under 5 with moderate-to-severe atopic dermatitis have IgE-mediated food allergy, but only 35% have clinically relevant food allergy 1
Essential Next Steps After Testing
- If testing is positive, perform a 4-6 week diagnostic elimination diet 1, 2
- Must confirm diagnosis with supervised oral food challenge under allergist guidance 1, 2
- Improvement during elimination may be coincidental or placebo effect 1
- Never start elimination diets based solely on positive tests without clinical correlation 1, 2
Critical Pitfalls to Avoid
Do Not Over-Test
- Extensive allergy testing without clear medical history leads to false positives that reflect sensitization rather than true allergy 1
- Avoid testing for foods without clinical suspicion 1
Do Not Under-Treat the Dermatitis
- Most common error: prescribing avoidance without treating the underlying atopic dermatitis 1
- Effective treatment remains skin care and topical therapies, even with confirmed food allergy 1
Do Not Implement Prolonged Elimination Without Confirmation
- Multiple elimination diets cause weight loss, growth delay, calcium deficiency, and nutritional deficiencies 1, 2
- Regular re-evaluation is necessary as most children develop tolerance over time 1, 2
When to Refer to Allergist
Referral is recommended for: