What is the best course of action for a 5-month-old infant with a new onset of a tiny red bump rash around the neck and on the back and sides, introduced to eggs 1 week ago and peanuts 36 hours ago, with no known changes to household products or other new foods?

Management of New Rash in 5-Month-Old Infant After Recent Food Introduction

This rash is most likely a benign, non-allergic skin condition common in infants, such as heat rash (miliaria rubra) or seborrheic dermatitis, rather than a food allergy, given the timing (36 hours post-peanut exposure) and distribution pattern (neck, back, sides—typical friction/heat areas). 1

Why This is Unlikely to be a Food Allergy

The timing and presentation argue strongly against peanut or egg allergy:

• Mild allergic symptoms from food typically appear within minutes to 2 hours of ingestion, not 36 hours later 1
• The NIAID guidelines specifically define mild food allergy symptoms as "a new rash" or "a few hives around the mouth or face"—not a generalized rash on neck, back, and sides appearing more than a day later 1
• The distribution pattern (neck, back, sides) is classic for heat rash or seborrheic dermatitis, which commonly affects skin folds and areas of friction in infants 2, 3
• True IgE-mediated food reactions manifest rapidly, typically within 10 minutes to 2 hours maximum 4

Most Likely Diagnoses to Consider

Based on the clinical presentation, prioritize these common infant rashes:

• Miliaria rubra (heat rash): Tiny red bumps in areas of heat retention and friction (neck, back) are pathognomonic; improves with cooling measures and loose clothing 2, 3
• Seborrheic dermatitis: Extremely common at this age, presents as red patches with or without scaling in skin folds including neck; may extend to trunk 2, 3
• Atopic dermatitis: Can present as red bumps but typically involves face and extensor surfaces first, with significant pruritus 5, 3
• Viral exanthem: Common in infants, often appears during or after minor viral illnesses that parents may not have noticed 6

Immediate Management Steps

Take the following actions to manage the rash and assess for progression:

• Cool the infant's environment and dress in loose, breathable cotton clothing to address potential heat rash 2, 3
• Apply gentle emollients (petroleum jelly or fragrance-free moisturizer) to affected areas twice daily 3
• Observe for 48-72 hours while documenting any changes in rash appearance, distribution, or associated symptoms 2, 5
• Continue peanut introduction as scheduled (2g peanut protein 3 times weekly) unless new concerning symptoms develop, since this rash is not consistent with peanut allergy 1

Red Flags Requiring Immediate Medical Attention

Seek emergency care immediately if any of these develop:

• Facial swelling, lip swelling, or tongue swelling 1
• Difficulty breathing, wheezing, or persistent cough 1
• Vomiting or diarrhea occurring within 2 hours of eating 1
• Hives spreading rapidly or appearing around the mouth/face within minutes to hours of food exposure 1
• Lethargy, irritability, or signs of systemic illness 2

When to Suspect True Food Allergy

Reconsider food allergy only if:

• A new rash appears within 10 minutes to 2 hours of the next peanut or egg exposure 1
• Hives develop specifically around the mouth or face during or immediately after eating 1
• The infant develops any respiratory or gastrointestinal symptoms temporally related to food ingestion 4
• There is a history of severe eczema (requiring prescription-strength topical steroids) which increases food allergy risk 1

Critical Pitfalls to Avoid

Do not make these common errors:

• Do not stop peanut introduction based on this rash alone, as premature avoidance increases future allergy risk and this presentation is inconsistent with allergic reaction 1
• Do not perform allergy testing at this point, as the clinical presentation does not warrant it and testing has poor positive predictive value in this context 1
• Do not attribute every infant rash to recent food introductions, as benign rashes are extremely common in this age group and usually unrelated to diet 2, 5, 3
• Do not use topical steroids without medical evaluation, as most infant rashes resolve spontaneously with conservative measures 2, 3

Follow-Up Plan

Schedule routine pediatric follow-up if:

• The rash persists beyond 1 week despite cooling measures and emollients 2, 3
• The rash becomes increasingly pruritic or the infant appears uncomfortable 5, 3
• New areas become involved or the rash changes character 5
• You have ongoing concerns about food introduction despite reassurance 1

The key principle: temporal relationship matters. Food allergies manifest rapidly (minutes to 2 hours), not 36 hours later, and this distribution pattern is classic for common benign infant dermatoses rather than allergic reactions 1, 2, 3.