Oral Food Challenge

An oral food challenge exposes a patient to escalating doses of a suspected food under medical observation. It is the reference method for confirming or excluding food allergy, including IgE-mediated and non-IgE-mediated reactions.

Prick testing and specific IgE may be positive without clinical allergy, and a history may be unreliable. The challenge answers whether the food actually causes reactions. A negative challenge at full dose excludes allergy to that food at the tested dose. A positive challenge confirms allergy and directs avoidance.

A double blind, placebo controlled challenge is the most reliable form. An open challenge is used where the history is clearly negative or the risk is low.

Non-IgE reactions. Some reactions, such as food protein induced enterocolitis and eosinophilic gastrointestinal disease, present with delayed or chronic symptoms and are not reliably detected by immediate reaction observation.

Indications. Uncertain history, discordant test results, and reintroduction of foods such as milk, egg or peanut in children after a mild reaction in the past.

Preparation and technique. Second generation antihistamines are stopped about 5 to 7 days beforehand, and first generation antihistamines about 2 to 3 days beforehand. The patient fasts for a short period before the challenge, typically 2 to 3 hours, and maintains a food diary. Doses are increased in fixed steps at set intervals, and the patient is observed throughout. The final observation period lasts at least 2 hours after the last dose.

Double blind method. Placebo and active doses are concealed, usually in capsules, and the observer is not told which is which.

Contraindications and precautions. Uncontrolled asthma, recent severe reaction to the food, uncontrolled cardiovascular disease, beta blocker use, active infection, and pregnancy, where the test is generally postponed. Lung function is checked beforehand. Challenges are generally deferred when the baseline FEV1 is below 70% of predicted, with the threshold confirmed locally.

Interpretation.

• A positive challenge shows an objective reaction within the observation window.
• A negative challenge at full dose shows tolerance at the tested dose.
• Subjective symptoms alone are interpreted cautiously, since anxiety can cause them.

Stopping criteria. Objective signs stop the challenge: urticaria, angioedema, vomiting, wheeze confirmed on examination, a fall in peak expiratory flow, or hypotension.

Causes of a positive challenge. IgE-mediated allergy, non-IgE-mediated reactions, and reactions to additives or contaminants in the test food.

Critical values and findings needing immediate action.

• Anaphylaxis can occur. Adrenaline, oxygen and a trained team must be present throughout the challenge.
• Any objective sign of airway or circulatory compromise requires immediate treatment.

Limitations and artefacts. Some reactions appear after the observation window, so a negative challenge does not fully exclude delayed or non-IgE reactions. Home reintroduction plans must account for this.

Follow up tests. Specific IgE testing, prick testing or component testing are used to define allergen sensitisation after a challenge. Ongoing reintroduction assessment uses further challenge at intervals set by the allergen and the patient's history.

Special groups. Children are commonly challenged in hospital, especially where the history is severe. In older adults, cardiovascular risk influences the choice of method and the dose. Pregnancy postpones most challenges.