What is the most appropriate investigation for a child with suspected atopic dermatitis who has not responded to emollient and low-dose corticosteroid treatment, with a suspected food allergy?

Most Appropriate Investigation for Suspected Food Allergy in Atopic Dermatitis

For a child with moderate-to-severe atopic dermatitis unresponsive to optimized topical therapy, the most appropriate initial investigation is specific IgE testing (Option A) or skin prick testing (Option B), with both being acceptable first-line approaches according to major guidelines.

Guideline-Based Testing Approach

When to Test

• Limited food allergy testing should be performed in children under 5 years with moderate-to-severe atopic dermatitis that persists despite optimized management and topical therapy 1, 2
• Testing should focus on the most common allergens: cow's milk, eggs, wheat, soy, and peanut 1, 2
• Broad panel allergy testing without clinical history is NOT recommended 1

Choice Between IgE Testing and Skin Prick Testing

Both are acceptable initial investigations with similar diagnostic characteristics 1:

Skin Prick Testing (SPT):

• High negative predictive value (>95%) 1, 2
• Low positive predictive value (40-60%) 1, 2
• Preferred when skin is relatively clear 1

Specific IgE Testing (Option A):

• Same diagnostic characteristics as SPT 1
• Preferable in cases with extensive eczematous lesions, prominent dermatographism, or recent antihistamine use 1, 2
• More practical in severe atopic dermatitis where skin involvement is extensive 1

Why Not Supervised Food Challenge Initially?

Supervised food challenge (Option C) is NOT the appropriate first investigation - it is the confirmatory test performed AFTER positive screening tests 1, 2:

• Positive skin or blood tests must be verified by controlled food challenges to confirm true allergy versus sensitization 1
• The double-blind, placebo-controlled food challenge is the gold standard for diagnosis, but comes AFTER initial screening 2
• A diagnostic elimination diet for 4-6 weeks followed by oral food challenge under allergist supervision is the proper sequence after positive screening tests 1, 2

Critical Interpretation Pitfalls

Understanding Test Results

• Negative tests effectively rule out IgE-mediated food allergy (>95% negative predictive value) 1, 2
• Positive tests only indicate sensitization, NOT necessarily clinical allergy - they require clinical correlation 1, 2
• False-positive rates are high (40-60%), so positive results must be confirmed with supervised challenges 1
• Measuring total IgE levels alone is NOT helpful in determining food allergy 1

Clinical Relevance

• Approximately 37% of children under 5 years with moderate-to-severe atopic dermatitis have IgE-mediated food allergy 1, 3
• However, the true frequency of food allergies causing isolated AD flares is probably low 1
• Food allergies may coexist with AD without necessarily triggering dermatitis flares 1

Practical Algorithm

1. Initial screening: Specific IgE testing OR skin prick testing for milk, egg, wheat, soy, peanut 1, 2

2. If negative: Food allergy effectively ruled out; focus on optimizing AD management 1

3. If positive:

   • Do NOT immediately eliminate foods 1, 2
   • Refer to allergist for interpretation 2
   • Consider 4-6 week diagnostic elimination diet 1, 2
   • Perform supervised oral food challenge to confirm clinical relevance 1, 2

4. Only eliminate foods with confirmed clinical allergy (positive challenge or convincing immediate reaction history) 1, 2

Important Caveats

• Empiric elimination diets without testing are harmful and can cause nutritional deficiencies, growth failure, and kwashiorkor 1
• Most children eventually develop tolerance to milk, egg, soy, and wheat 1
• Effective AD treatment remains skin care and topical therapies even with confirmed food allergy 2
• A retrospective study showed 84-93% of avoided foods based on sensitization alone could be safely reintroduced 1