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 in whom a food allergy is suspected?

Most Appropriate Investigation for Suspected Food Allergy in Atopic Dermatitis

The most appropriate investigation is a supervised food challenge (Option C), but only after initial screening with skin prick testing or specific IgE levels to identify potential allergens, followed by a diagnostic elimination diet if testing is positive. 1, 2

Algorithmic Diagnostic Approach

Step 1: Optimize Topical Therapy First

• Before pursuing food allergy investigation, ensure the child has received adequate treatment with appropriate-potency topical corticosteroids and sufficient emollient use, as many children have inadequately treated atopic dermatitis rather than food allergy. 1, 2
• Ask parents to keep a food diary for one month documenting foods eaten before eczema flare-ups to establish temporal correlation. 3

Step 2: Initial Allergy Testing (If Criteria Met)

• In children under 5 years with moderate-to-severe atopic dermatitis persisting despite optimized topical management, perform limited testing for cow's milk, eggs, wheat, soy, and peanuts using either skin prick testing or specific IgE levels. 3, 1, 2
• Skin prick testing (Option B) or specific IgE levels (Option A) serve as the initial screening tools, not the definitive diagnostic test. 3, 2
• These tests have high negative predictive value (>95%) but poor positive predictive value (40-60%), meaning negative results effectively rule out food allergy, but positive results only indicate sensitization, not true allergy. 3, 2

Step 3: Elimination Diet (If Testing Positive)

• If skin prick testing or IgE testing is positive AND there is consistent clinical correlation with food exposure, implement a diagnostic elimination diet for 4-6 weeks with the suspected food items. 1, 2
• Do not start elimination diets based solely on positive tests without clinical correlation, as 50-90% of presumed food allergies based on symptoms or positive tests are not true allergies. 2

Step 4: Supervised Food Challenge (Gold Standard)

• A supervised oral food challenge under allergist guidance is essential to confirm the diagnosis, as improvement during elimination may be coincidental or placebo effect. 1, 2
• The double-blind, placebo-controlled food challenge is the gold standard, though open-label challenges are acceptable in clinical practice. 1, 2
• Evaluate skin status using an established score (SCORAD or EASI) at 24 hours and later after the challenge. 1

Why Each Option Is Right or Wrong

Option A (IgE levels): This is a reasonable initial screening test but insufficient alone, as it only indicates sensitization with poor positive predictive value (40-60%). 3, 2

Option B (Prick skin test): This is also a reasonable initial screening test with the same limitations as IgE testing—high negative predictive value but poor positive predictive value. 3, 2

Option C (Supervised food challenge): This is the definitive diagnostic test and gold standard, but should follow initial screening tests and elimination diet rather than being performed first. 1, 2, 4

Option D (Food elimination): This is an intermediate step that should only be done after positive screening tests with clinical correlation, and must be followed by supervised challenge to confirm diagnosis. 1, 2

Critical Pitfalls to Avoid

• Do not perform broad panel allergy testing without clear clinical history, as positive results may only reflect sensitization rather than true allergy. 1
• Avoid prescribing avoidance without treating the underlying atopic dermatitis, as this is incorrect management. 3, 1
• Prolonged and multiple elimination diets without confirmation can cause weight loss, growth delay, calcium deficiency, hypovitaminosis, and kwashiorkor. 1, 2
• Food allergy is relevant in only 35% of children with moderate-to-severe atopic dermatitis, so do not assume all cases are food-related. 3, 1

When to Refer to Allergist

• Refer to an allergist for longitudinal care, supervised oral food challenges, formulation of emergency anaphylaxis action plans, and prevention of unnecessary dietary restrictions. 1
• The allergist will coordinate the diagnostic elimination diet and supervised challenge protocol. 1

Practical Answer to the Question

In the clinical scenario described, the correct sequence is: perform skin prick testing or specific IgE levels first (Options A or B), then proceed to elimination diet if positive with clinical correlation, and finally confirm with supervised food challenge (Option C). If forced to choose a single "most appropriate" investigation from the options, supervised food challenge (Option C) is the gold standard for definitive diagnosis, though it requires prior screening in practice. 1, 2, 4