Allergy Testing in Infants: Age-Specific Approach
For infants with severe eczema or egg allergy, perform peanut-specific IgE testing or skin prick testing as early as 4-6 months of age before introducing peanut-containing foods. 1
Risk-Stratified Testing Strategy
High-Risk Infants (Severe Eczema and/or Egg Allergy)
Testing should be performed before peanut introduction at 4-6 months:
- Peanut-specific IgE is the preferred initial test in primary care settings (family medicine, pediatrics, dermatology) where skin prick testing is not routinely available 1
- A peanut-specific IgE <0.35 kUA/L has strong negative predictive value and allows home introduction of peanut 1
- A peanut-specific IgE ≥0.35 kUA/L requires specialist referral for further evaluation and possible skin prick testing 1
Skin prick testing interpretation for high-risk infants:
- Wheal diameter ≤2 mm above saline control: introduce peanut at home or supervised in office 1
- Wheal diameter 3-7 mm: moderate-to-high risk, requires supervised feeding or oral food challenge 1
- Wheal diameter ≥8 mm: likely allergic, requires specialist management 1
Moderate-Risk Infants (Mild-to-Moderate Eczema)
- Introduce peanut-containing foods at approximately 6 months without prior testing 2
- Testing is not routinely required unless clinical concerns arise 2
Low-Risk Infants (No Eczema or Food Allergy)
- Introduce peanut-containing foods with other age-appropriate solid foods without testing 2
Age-Specific Testing Considerations
Infants <2 years require lower diagnostic thresholds:
- For cow's milk: SPT wheal ≥6 mm is diagnostic (vs. ≥8 mm in older children) 3
- For egg: SPT wheal ≥5 mm is diagnostic (vs. ≥7 mm in older children) 3
- For peanut: SPT wheal ≥4 mm is diagnostic (vs. ≥8 mm in older children) 3
At 1 year of age, 95% positive predictive values are:
- Peanut SPT ≥8 mm or peanut-specific IgE ≥34 kUA/L 4
- Egg SPT ≥4 mm or egg-specific IgE ≥1.7 kUA/L 4
- Sesame SPT ≥8 mm 4
Critical Testing Principles for Infants
Agreement between SPT and specific IgE varies by age:
- Agreement is better in infants <6 months compared to ≥6 months for egg white (κ=0.79 vs. 0.54) 5
- SPT shows substantial agreement with specific IgE for egg white (κ=0.62-0.69) and moderate agreement for cow's milk (κ=0.34-0.47) in infants <12 months 5
- Both tests should be used when possible, as agreement is imperfect in early childhood 6
Infants <6 months who have never been exposed may have negative or below-threshold SPT results initially, but these often become positive by the second year of life 3
What NOT to Do
Do not order food allergen panels or test for multiple foods beyond peanut in high-risk infants - these have poor positive predictive value and lead to misdiagnosis, overdiagnosis, and unnecessary dietary restrictions 1
Do not test infants without clinical symptoms or risk factors - a positive test without clinical history does not establish food allergy diagnosis 7
Do not rely solely on testing to diagnose food allergy - diagnosis requires both sensitization (positive test) AND clinical symptoms with food exposure 7, 8
Testing for Suspected Food Allergy in Symptomatic Infants
Children <5 years with moderate-to-severe atopic dermatitis should be evaluated for:
- Cow's milk, egg, peanut, wheat, and soy 8
- Testing is indicated if: persistent dermatitis despite optimized topical therapy OR reliable history of immediate reaction after specific food ingestion 8
- Optimize skin care with moisturizers and topical steroids before attributing symptoms to food allergy 9
Beware of false positives - ensure children are clinically allergic and not just sensitized before implementing dietary restrictions 8