Food allergy diagnosis and oral food challenge — formative SAQs

SAQ 1: Diagnostic approach to a child with suspected peanut allergy (10 marks, 15 minutes)

Stem: A 14-month-old boy with a history of moderate atopic dermatitis developed perioral urticaria and one episode of vomiting 10 minutes after eating peanut butter for the first time. His mother stopped the exposure; symptoms resolved over 30 minutes without treatment. She asks you to "test him for food allergies." ^[1]

(a) What six elements of a structured reaction history would you elicit? (3 marks) ^[1]

1. What — the specific food and its form (raw, cooked, processed — peanut butter in this case). ^[1]
2. How much — estimated dose ingested before symptoms. ^[1]
3. When — time from ingestion to symptom onset (10 minutes — consistent with IgE-mediated). ^[1]
4. What happened — specific symptoms by organ system (cutaneous: urticaria; GI: vomiting; check for respiratory, cardiovascular involvement). ^[1]
5. Reproducibility — has this happened before with the same or related foods? ^[1]
6. Cofactors — exercise, NSAID use, intercurrent illness, alcohol (adolescents) that might potentiate reaction. ^[1]

(b) What is the appropriate diagnostic testing strategy, and what thresholds would guide interpretation? (4 marks) ^[1]

Perform a skin prick test (SPT) to peanut only — not a broad food allergy panel (broad panels generate false positives in atopic children and cause harm from unnecessary dietary restriction). ^[1][4] Alternatively or additionally, serum peanut-specific IgE. ^[2]

SPT 95% positive predictive value (PPV) threshold for peanut in young children: wheal diameter 8 mm or greater — above this, approximately 95% will react on oral food challenge. ^[1][4] Serum peanut-specific IgE 95% PPV: 14 kU/L or greater in children. ^[1] A positive test with a convincing history confirms the diagnosis. ^[1]

If the result is ambiguous (SPT 3-7 mm or sIgE below 14), component-resolved diagnostics for Ara h 2 should be requested — Ara h 2 is the most predictive single component for clinical peanut allergy and can reduce the need for oral food challenge. ^[4][2]

(c) When would you perform an oral food challenge, and describe the key elements of the protocol? (3 marks) ^[3]

OFC is indicated when history and test results are discordant or inconclusive, or to assess for resolution of a previously confirmed allergy. ^[1][3] Contraindicated if uncontrolled asthma or acute illness. ^[3]

Protocol: escalating doses of challenge food at 15-20 minute intervals to a total serving-equivalent dose (4-8 g protein for peanut); assess for objective symptoms after each dose; stop on any objective reaction or completion of total dose; observe for 1-2 hours after final dose. ^[3][1]

Safety: performed under medical supervision with IV access (for high-risk), IM adrenaline (0.01 mg/kg of 1:1000 into anterolateral thigh), oxygen, nebulised bronchodilators, and IV fluids immediately available. ^[3][1] Formats: DBPCFC is gold standard (eliminates bias); open challenge appropriate for routine clinical use when expected reaction is objective. ^[3]