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

Most Appropriate Investigation for Suspected Food Allergy in Treatment-Resistant Atopic Dermatitis

In a child with moderate-to-severe atopic dermatitis unresponsive to optimized topical therapy, perform limited skin prick testing (SPT) for common food allergens (milk, eggs, wheat, soy, and peanuts) rather than empirical elimination or IgE levels alone. 1, 2

Why Skin Prick Testing is the Preferred Initial Investigation

Skin prick testing should be the first-line diagnostic test because it has superior diagnostic characteristics compared to serum IgE testing, with a high negative predictive value (>95%) that effectively rules out food allergy when negative. 1, 2 The test provides rapid results and is more cost-effective than serum testing. 3

When to Use Serum-Specific IgE Instead

Serum-specific IgE testing is appropriate only when SPT cannot be performed due to: 1, 2

• Extensive eczematous lesions covering potential test sites
• Prominent dermatographism that would interfere with interpretation
• Recent antihistamine use (within 3-7 days depending on the agent)

Why Empirical Elimination is Inappropriate

Do not start empirical food elimination without diagnostic testing first. 2 This approach is problematic because:

• Improvement during elimination may be coincidental or placebo effect, not true food allergy 2
• Prolonged elimination diets cause nutritional deficiencies, growth delay, calcium deficiency, and weight loss 1, 2
• Only approximately 35% of children with moderate-to-severe atopic dermatitis actually have food allergy 1
• The underlying atopic dermatitis often remains untreated when providers focus solely on food elimination 1, 2

Critical Pitfall to Avoid

The most common error is prescribing avoidance without treating the underlying atopic dermatitis. 1, 2 Before attributing symptoms to food allergy, you must first optimize topical corticosteroid therapy and ensure adequate emollient use. 2 Many children will improve with proper topical management alone, making food allergy investigation unnecessary.

The Complete Diagnostic Algorithm

Step 1: Confirm Testing Criteria Are Met

This child meets criteria for testing because they have: 1, 2

• Age <5 years with moderate-to-severe atopic dermatitis
• Persistent disease despite optimized topical therapy (emollients + appropriate-potency corticosteroids)

Step 2: Perform Limited SPT

Test only the five most relevant allergens for this age group: 1, 2

• Cow's milk
• Eggs
• Wheat
• Soy
• Peanuts

Step 3: Interpret Results Correctly

• Negative SPT (>95% negative predictive value): Food allergy is effectively ruled out; focus on optimizing atopic dermatitis treatment 1, 2
• Positive SPT: This indicates sensitization only, NOT confirmed allergy (positive predictive value only 40-60%) 1, 2

Step 4: Confirm Positive Results

If SPT is positive, proceed with: 1, 2, 3

• 4-6 week diagnostic elimination diet of the specific food(s)
• Supervised oral food challenge under allergist guidance to confirm true allergy
• The double-blind, placebo-controlled food challenge (DBPCFC) is the gold standard 1, 3

When to Refer to Allergist

Refer to an allergist for: 1, 2

• Performing and interpreting SPT
• Supervising oral food challenges
• Managing confirmed food allergies with emergency action plans
• Preventing unnecessary dietary restrictions
• Longitudinal follow-up, as most children develop tolerance over time 2, 3

Understanding What Food Allergy Actually Causes in AD

Food allergy typically causes immediate reactions (within 2 hours) including urticaria, angioedema, flushing, or gastrointestinal/respiratory symptoms—not primarily eczematous flares. 1 Delayed eczematous reactions can occur 6-48 hours later, but this is less common. 1 This distinction is important because the child may have both conditions (AD and food allergy) without one necessarily causing the other.

Why Not Measure Total IgE Levels

Measuring total serum IgE levels alone is not helpful in determining food allergy and should not be performed. 1 Only allergen-specific IgE testing has diagnostic value, and even then, SPT is preferred when feasible.