What tests can allergists do for a 7-month-old infant with severe eczema to diagnose environmental and food allergies?

Allergy Testing for a 7-Month-Old Infant with Severe Eczema

For a 7-month-old infant with severe eczema, allergists should perform skin prick testing (SPT) or food-specific IgE blood testing for the five most common food allergens: cow's milk, eggs, wheat, soy, and peanut. 1

When Testing is Indicated

Testing is appropriate for this infant because they meet specific criteria:

• Age under 5 years with moderate to severe atopic dermatitis 1
• Persistent disease despite optimized topical therapy (appropriate-potency topical corticosteroids and adequate emollient use) 1, 2
• Testing should be limited to the five common allergens listed above, not broad panel testing 1, 2

The rationale is strong: infants with severe eczema have up to 37% prevalence of IgE-mediated food allergy, with those requiring doctor-prescribed topical corticosteroids having a 50.8% risk of challenge-proven food allergy. 3 Egg white is by far the most common positive test in this population, with 80-90% of infants with moderate to severe eczema testing positive. 4

Primary Testing Options

Skin Prick Testing (SPT)

• First-line test for identifying IgE-mediated food allergies 1
• High negative predictive value (>95%) but low positive predictive value (40-60%) 1
• Negative results effectively rule out food allergy, but positive results only indicate sensitization, not confirmed allergy 1, 2
• Results available within 15-20 minutes 1

Food-Specific IgE Blood Testing

• Alternative to SPT, particularly useful when:
  • Extensive eczematous lesions cover potential testing sites 1
  • Prominent dermatographism is present 1
  • Recent antihistamine use prevents SPT 1
• Same predictive values as SPT: high negative predictive value (>95%), low positive predictive value (40-60%) 1
• Larger wheal sizes (>8-10 mm) or higher specific IgE levels correlate with greater likelihood of true allergy 1

Tests That Should NOT Be Performed

• Intradermal testing: Not recommended for food allergy diagnosis 1
• Total serum IgE: Should not be used routinely for food allergy diagnosis 1
• Atopy patch testing (APT): Should not be used in routine evaluation of non-contact food allergy 1
• Broad panel allergy testing: Not recommended without specific clinical history, as it leads to false positives and unnecessary dietary restrictions 1, 2, 5

Critical Next Steps After Testing

Positive test results alone are NOT diagnostic of food allergy. 1 The following steps are essential:

1. Diagnostic elimination diet for 4-6 weeks if testing is positive 2
2. Supervised oral food challenge under allergist guidance to confirm diagnosis, as improvement during elimination may be coincidental or placebo effect 2, 5
3. Double-blind placebo-controlled food challenge (DBPCFC) is the gold standard, though single-blind or open challenges may be diagnostic under certain circumstances 1, 2

Environmental Allergy Testing

For environmental allergens in a 7-month-old:

• Generally not indicated at this age, as environmental allergies typically develop later in childhood 6
• Skin prick testing to inhalant allergens is typically performed after 3 years of age 6
• Early onset eczema is associated with increased risk of later sensitization to inhalant allergens, but testing at 7 months is premature 6

Common Pitfalls to Avoid

• Never prescribe avoidance based solely on positive test results without confirmatory oral food challenge 1, 2
• Avoid multiple or prolonged elimination diets without documented, clinically relevant food allergies, as this leads to weight loss, poor growth, calcium deficiency, and other nutritional deficiencies 1, 2
• Do not attribute all symptoms to food allergy without first optimizing topical eczema treatment, as many children have inadequately treated atopic dermatitis, not food allergy 2
• Never order screening panels without clinical suspicion, as 50-90% of presumed food allergies are not true allergies 1, 5

Special Considerations for Peanut Testing

Given this infant's severe eczema, peanut testing is particularly important:

• Infants with severe eczema are 11 times more likely to develop peanut allergy by 12 months compared to infants without eczema 3
• If SPT wheal size is 0-2 mm for peanut, introduction of age-appropriate peanut-containing foods at home or supervised in-office introduction is recommended 1
• If SPT wheal size is 3-7 mm, graded oral peanut challenge or supervised in-office introduction should be performed 1
• If SPT wheal size is ≥8 mm, avoidance with continued evaluation and monitoring is recommended 1