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

Diagnostic Approach for Suspected Food Allergy in Atopic Dermatitis

In a child with suspected atopic dermatitis not responding to emollients and low-dose corticosteroids where food allergy is suspected, the most appropriate investigation is skin prick testing (SPT) or specific IgE testing for the most common food allergens (cow's milk, egg, peanut, wheat, and soy), followed by a diagnostic elimination diet and oral food challenge to confirm clinical relevance. 1, 2

Initial Testing Strategy

Who Should Be Tested

• Children under 5 years with moderate-to-severe atopic dermatitis that persists despite optimized topical treatment should be considered for limited food allergy testing 1, 3
• Testing is also indicated if there is a reliable history of immediate reaction after ingesting a specific food 3, 1
• Approximately 30-37% of children with moderate-to-severe atopic dermatitis have IgE-mediated food allergy 4, 5, 6

Which Foods to Test

• Limit testing to the five most common allergens in young children: cow's milk, eggs, wheat, soy, and peanuts 1, 4
• These foods account for >90% of food allergies in children with atopic dermatitis 7
• Extensive allergy testing without clear medical history should be avoided, as positive results may only reflect sensitization rather than true allergy 1

Testing Methods

Skin Prick Testing (Preferred)

• SPT has a high negative predictive value (>95%) but lower positive predictive value (40-60%) 1, 2
• This means a negative test effectively rules out IgE-mediated food allergy, but a positive test only indicates sensitization, not necessarily clinical allergy 1, 2
• SPT should be performed with both negative and positive histamine controls, in duplicate, with maximum wheal diameter measured 4

Specific IgE Testing (Alternative)

• Can be used as an alternative to SPT, especially when widespread skin lesions or marked dermatographism make skin testing difficult 1
• Has similar limitations as SPT: poor positive predictive value (40-60%) 2
• 50-90% of presumed food allergies based on positive tests alone are not true allergies 2

Critical Next Steps After Testing

Diagnostic Elimination Diet

• If testing is positive, perform a diagnostic elimination diet for 4-6 weeks with the suspected food items 1, 2
• Never start an elimination diet based solely on positive skin tests or IgE levels without clinical correlation 2, 4
• The elimination diet should only be implemented if there is consistent correlation between food exposure and dermatitis flares 2

Oral Food Challenge (Essential for Confirmation)

• An oral food challenge under allergist guidance is essential to confirm the diagnosis, as improvement during elimination may be coincidental or placebo effect 2, 3
• Double-blind, placebo-controlled food challenge (DBPCFC) is the gold standard, though open-label challenges are acceptable in clinical practice 1, 2
• This step prevents unnecessary long-term dietary restrictions based on false-positive testing 2, 4

Critical Pitfalls to Avoid

Do Not Over-Test or Over-Restrict

• Food allergies are relevant triggers in only a small subset of AD patients (usually moderate-to-severe disease) 2
• Excessively restrictive diets can cause serious harm including weight loss, poor growth, calcium deficiency, hypovitaminosis, and kwashiorkor 1, 2, 4
• Prolonged and multiple elimination diets without confirmed clinical relevance are harmful 1

Do Not Neglect Topical Treatment

• Effective treatment of atopic dermatitis remains skin care and topical therapies, even in the presence of confirmed food allergy 1
• Often, avoidance is prescribed and no treatment for the underlying atopic dermatitis is provided, which is incorrect management 3
• Optimize topical corticosteroid therapy (consider medium-potency TCS if low-dose failed) and ensure adequate emollient use before attributing all symptoms to food allergy 3, 4

Understand Test Limitations

• Positive tests only indicate sensitization; clinical relevance must be established through elimination and challenge 2, 4
• The older allergy literature overspecifies the relationship between food-specific IgE and food allergy in this population 3
• Care must be taken to not overtest or interpret results too literally 3

When to Refer to Allergist

• Referral to an allergist is recommended for longitudinal care, diagnostic testing, formulation of emergency anaphylaxis action plan, and supervised oral food challenges 3
• The allergist will help facilitate proper elimination diets under specialist supervision and prevent unnecessary dietary restrictions 3, 6
• Most children with food allergies develop tolerance over time, and regular re-evaluation by an allergist is necessary 1