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]

SAQ 2: Oral food challenge — safety, interpretation, and counselling (10 marks, 15 minutes)

Stem: A 5-year-old girl with egg allergy since age 2 has a current SPT wheal of 4 mm to egg (previously 8 mm) and serum egg-specific IgE of 3.2 kU/L (previously 18 kU/L). Her parents ask whether she may have outgrown the allergy. You plan an oral food challenge. [4]

(a) What pre-challenge assessment is required before proceeding? (3 marks) [3]

1. Asthma assessment — uncontrolled asthma is a contraindication; check for current symptoms, recent oral steroid use, peak flow/spirometry if age-appropriate. Postpone until optimised if poorly controlled. [3][1]
2. General health — no acute illness (especially respiratory or GI infection) in the preceding 1-2 weeks; no antihistamines for 5-7 days before challenge. [3]
3. Informed consent — discuss the purpose, risks (10-15% reaction rate), what happens during a reaction, and the observation period. Document the discussion. [3][1]
4. Baseline observations — vital signs, weight (for dose calculation and adrenaline dosing), skin examination to establish baseline. [3]

(b) During the challenge, after the third escalating dose of egg, she develops generalised urticaria and cough. Describe your immediate management. (4 marks) [3]

1. Stop the challenge immediately. [3][1]
2. Assess ABCDE — airway patency, breathing (wheeze, respiratory rate, oxygen saturation), circulation (heart rate, blood pressure, capillary refill), disability (conscious level), exposure (skin examination). [1][3]
3. If anaphylaxis criteria met (rapid onset involving skin/mucosa plus respiratory or cardiovascular compromise): IM adrenaline 0.01 mg/kg of 1:1000 into the anterolateral thigh (vastus lateralis). Repeat every 5 minutes if no improvement. Lie the child flat with legs elevated. [3][1]
4. If urticaria and cough without airway or cardiovascular compromise: observe closely; consider oral antihistamine; be prepared to escalate to IM adrenaline if progression. [1][3]
5. Oxygen if hypoxaemic; nebulised salbutamol if wheeze; IV fluids if hypotensive. [1]
6. Observe for minimum 2 hours after symptom resolution (biphasic reaction risk). [3]

(c) After a negative baked-egg challenge, what advice do you give the family? (3 marks) [4]

1. The child can safely include baked egg (in muffins, cakes, biscuits — foods where egg is baked at high temperature as part of a wheat matrix) in her diet. [4][2]
2. She should consume baked egg regularly (2-3 times per week) to maintain tolerance and accelerate the acquisition of full egg tolerance. [2][4]
3. She should continue to avoid loosely cooked and raw egg (scrambled, boiled, meringue, mayonnaise) until a further challenge confirms tolerance to these forms. [2]
4. Provide a written plan and advise the family to introduce baked egg at home within the following days. [1]
5. Schedule follow-up to reassess for full egg tolerance with a further challenge (to less-heated forms) in 12-18 months. [4]