Oral food challenges are procedures conducted to make an accurate diagnosis of immediate, and occasionally delayed, adverse reactions to foods. The timing of the challenge is carefully chosen based on the individual patient history and the results of skin prick tests and food specific serum IgE values. The type of the challenge is determined by the history, the age of the patient, and the likelihood of encountering subjective reactions. The food challenge requires preparation of the patient for the procedure and preparation of the office for the organized conduct of the challenge, for a careful assessment of the symptoms and signs and the treatment of reactions. The starting dose, the escalation of the dosing, and the intervals between doses are determined based on experience and the patient’s history. The interpretation of the results of the challenge and arragements for follow-up after a challenge are important. A negative oral food challenge result allows introduction of the food into the diet, whereas a positive oral food challenge result provides a sound basis for continued avoidance of the food.
Diagnostic oral food challenges (OFCs), introduced in clinical practice by May 1976 in the form of double-blind, placebo-controlled food challenges (DBPCFCs), are procedures that may be used to establish definitively whether a food is the cause of adverse reactions. They are used as clinically indicated, either at initial diagnosis or during follow-up. This article is intended to provide a practical and comprehensive guide to aid allergists/immunologists, who are uniquely qualified to perform diagnostic OFCs, in the use of the procedure for patient care in a scientifically sound, safe, and practical manner. It describes approaches used by allergists/immunologists in the United States and incorporates critically reviewed published international experience. Additional resources are “Food Allergy: A Practice Parameter” and “A Health Professional’s Guide to Office Food Challenges.”
Types of OFC Oral Food Challenges
There are various types of OFC that may be clinically indicated, including open, single-blind, or double-blind, placebo-controlled. The choice of the type of OFC is based on clinical assessment of the potential for bias in the interpretation of the results
Decision process regarding selection of open vs blind OFCs.Blinded portions of the OFC must always be followed by an open feeding with an age-appropriate serving of food in its natural form or the least cooked/baked/processed form of food that will be incorporated into the patient’s diet at home.
Open OFC is an unmasked, unblinded feeding with a food in its natural form that is usually done if objective symptoms such as urticaria and wheezing are anticipated and concern for bias is low. It is usually used for screening challenges in the office setting, according to a simplified protocol of gradual feeding with an age-appropriate serving of food, followed by an observation period of about 1 to 2 hours. It is simple to plan and reproduces the natural exposure in quantity and method of preparation. However, it has the highest potential for bias, which may depend on age, personality, and type of symptoms. Whereas a clearly negative open OFC rules out reactions to the food, a positive result with only subjective symptoms, such as itchy mouth and nausea, may need to be confirmed by a blinded challenge. Open OFC is a cost-efficient procedure that saves substantial time and resources, particularly because only about one third of suspected foods result in a positive challenge. It is thus considered a reasonable first choice to evaluate an adverse reaction to a food where the need for OFC has been established.
Open OFC can also be done using a slow dose-escalation schedule as outlined in the blinded OFC sections of this article. This may be a useful approach for OFC performed for clinical purposes when concern for bias is low but concern for a severe reaction is high.
Blinding and masking by mixing the challenge food with a masking vehicle or placing food in opaque capsules reduces bias. In the single-blind OFC, the observer but not the patient knows the food being tested. In the double-blind OFC, challenge material is provided by a third party, such as a dietitian, whereas the patient, the patient’s family, and the observer are unaware of when the test food is given. Thus, bias is minimized. Placebo-controlled challenges may be administered in both single-blind and double-blind fashion.
Single-blind OFC may be conducted with or without placebo, depending on the physician’s judgment of the potential for subjective symptoms and the patient’s anxiety. In the single-blind OFC without placebo, the patient is told that test food may or may not be served during the challenge; if 2 foods are tested on the same day, the sequence of the foods is not revealed to the patient. If the food is tolerated without any reaction or results in objective symptoms, valid conclusions can be drawn without the need for a placebo challenge. If multiple foods are being tested, the patient or parent is informed at the beginning that a series of tests will be carried out on several visits and that the tested foods will not be revealed until all challenges are completed. Single-blind OFC can be reliable in most cases evaluated in clinical practice; however, bias on the part of the patient may occur if the observer’s attitude is inconsistent during all challenges. Single-blind OFC does not eliminate observer bias. In patients suspected of having a psychological response, the placebo might be tested first. If symptoms develop, foods that give a positive result should be retested for reproducibility in a DBPCFC.
If a placebo-controlled OFC is undertaken, the food should be administered in a form that would not allow its differentiation from the placebo. A single-blind, placebo-controlled OFC consists of 2 sessions, 1 with active food and 1 with placebo, completed on 1 day with at least a 2-hour period separating the 2 sessions or on separate days. Alternatively, if such a prolonged challenge procedure is not practical but subjective symptoms are anticipated, placebo doses may be interspersed with real food doses during 1 session to help with the interpretation of vague, subjective symptoms. However, in dubious cases, and in patients presenting with subjective symptoms only, a protocol with repeated challenges should be applied by using 3 verum plus 2 or 3 placebo challenges. For patients reporting delayed onset of symptoms, sessions of blinded OFC are separated by several days or weeks.
After a negative blinded challenge, an open feeding with the tested food in its natural form should be undertaken approximately 2 hours after completing the final negative blind session or on a separate day, and followed by an additional observation period of 2 hours or less, depending on the clinical circumstances. These recommendations are based on the 3% possibility of detecting a reaction to an open feeding in children (no data available for adults), possibly because of the larger amount of food ingested during an open feeding, effects of the vehicle matrix on allergen accessibility and absorption, or subclinical reactions caused by gradual administration. Tolerance is proven when the food has been returned to the diet and consumed in its usual form of preparation and quantities.
Double–blind, placebo-controlled OFC
A DBPCFC is the most rigorous challenge design for the diagnosis of adverse food reactions. Test foods and placebo are prepared and coded by a third party not involved in evaluating the patient, minimizing both patient and observer bias. The sequence of sessions administering either the test food or the placebo is random. On completion of the challenges, the code is broken, and results are discussed with the patient or parent. With challenges to multiple foods, results are discussed after completing all the challenges. Open feedings with tested foods in their natural form should be done before concluding there is tolerance. DBPCFC is used for research studies and for selected cases in clinical practice.
Challenges to multiple foods
For patients’ convenience, open or single-blind challenges to more than 1 food may be done on the same day, separated by a break of approximately 2 hours, during which a light lunch may be consumed. If past reactions occurred later than 2 hours, multiple foods should not be tested on 1 day, unless there is a need to exclude immediate reactions in a patient who has a history of only delayed reactions to the test food, such as patients with allergic eosinophilic esophagitis who have positive SPT or detectable serum food-specific IgE. After a negative OFC, test foods should be reintroduced 1 at a time at home.
In the case of cross-reactive foods such as tree nuts, fish, or shellfish, challenging to multiple foods during 1 session might be considered for practical reasons, especially if patients have a low risk of reacting. Patients with a documented allergy to a food usually avoid all related foods. Patients allergic to 1 tree nut often choose to avoid all tree nuts without knowledge of their clinical reactivity because of the high risk of cross-contamination during commercial food processing. In a mixed food challenge, several nuts may be mixed together or ingested in sequence during 1 OFC session. If the patient reacts to the mixed food challenge, avoidance of all tested foods is recommended, or subsequent OFC to individual food may be conducted to identify the food allergens precisely. It is to be cautioned, however, that patients may not be able to ingest the recommended amount of each individual food during a mixed OFC. Considering these factors, several sessions of mixed challenges may be necessary to establish tolerance to all foods from the same food group (eg, all tree nuts) in both children and adults.
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