Food Allergic Patient Evaluation - Food Challenges

The goal of performing an oral food challenge is to document the presence or lack of clinical reactivity to a food. Oral food challenges are categorized into open, single-blind, placebo-controlled, or double-blind, placebo- controlled, depending upon who is aware of the contents of each dose given during the challenge.

The type of oral food challenge selected for use depends upon the expected need to control for patient or observer bias. In a double-blind, placebo-controlled food challenge (DBPCFC) neither the patient nor the medical team involved in administering the challenge is aware of the contents of the challenge.

The DBPCFC remains the gold standard for diagnosing food allergy, as it best controls for both patient and observer bias. Thus, the DBPCFC is the challenge of choice in the research setting. In a single-blind, placebo-controlled food challenge (SBPCFC) the medical staff knows the contents of challenge doses, but the patient does not.

SBPCFCs are performed to eliminate bias on the part of the patient and the patient’s family. In an open food challenge (OFC), because the food is provided in its usual edible form, both the patient and medical staff knows which and how much food is being eaten.

A benefit of the OFC is the relative ease of performance, as masking the challenge food is unnecessary, thereby significantly reducing preparation time. In most clinical situations an OFC suffices when objective symptoms are used to determine if the challenge is positive. Oral food challenges are performed to address a variety of clinical questions.

When the food responsible for a reaction remains unclear, even after a thorough history and attempts to document sensitization, or when more than one food is implicated based on the history and test results, food challenges are indicated to determine which, if any, of the suspected foods cause symptoms.

Accurately identifying the causative food prevents future reactions and avoids the needless elimination of foods from the diet. Food challenges are also performed to prove that a food is not or is no longer the cause of symptoms. An example is the patient who has been inaccurately labeled as food allergic, despite an unconvincing history or suspicious skin test or immunoassay results.

A food challenge is indicated for reassurance that the food can be safely returned to the diet. The majority of children, who as infants and toddlers were allergic to milk, egg, soy, or wheat, develop tolerance to these foods as they age. More recent studies have shown that 20% of children with allergic reactions to peanut in the first years of life outgrow their sensitivity.

Children determined to be allergic to a specific food at an early age, and during the course of their initial evaluation have evidence of sensitization to other foods they have yet to eat, are often kept on diets eliminating these foods until they are older.

A thorough exposure history, combined with information obtained from skin testing and immunoassay for the level of food allergen-specific IgE, is used to determine if and when to challenge children to these foods. Carefully performed food challenges safely determine when these foods can be added to the diet.

When immune mechanisms other than IgE-mediated sensitivity are suspected, as exemplified with FPIES, a food challenge may be the only accurate means of verifying the diagnosis. Contraindications to the performance of food challenges are relatively few.

The performance of a food challenge is not advised in a patient with a history of a previous life threatening reaction and no evidence to suggest a significant decrease in the patient’s level of sensitivity to the food. Challenging a patient with unstable asthma is ill advised.

Patients with severe eczema should have their eczema under control before being challenged. When a previous reaction was severe and the obvious causative food is rarely encountered in the diet, or if the patient dislikes the food and, if given the opportunity, would chose to avoid it, the potential benefit is unlikely to outweigh the risks of challenge.

Patients frightened by the consideration of a food challenge often benefit from working with a psychosocial clinician before a food challenge is performed. Decisions about who should be challenged are finalized only after a thorough discussion with the patient or the patient’s family regarding the reasons for challenge, in addition to a review of the potential benefits and risks.

Although significant risk can be associated with the performance of oral food challenges, a retrospective analysis of 253 failed food challenges to the common food allergens (milk, egg, peanut, soy, or wheat), performed in a tertiary care center, along with the experience of other centers that routinely perform these challenges, provide evidence that controlled oral food challenges are safe when performed in a medical setting, with the necessary medications and equipment, in addition to personnel experienced in the treatment of anaphylaxis.

Before beginning a challenge, the history of previous suspected reactions to the food is reviewed and an interim history is taken to verify that the patient is medically stable. The basic structure of a food challenge involves feeding gradually increasing doses of the suspected food at predetermined time intervals, until objective symptoms occur or a normal portion of the food ingested openly is tolerated.

In blinded studies the challenge food is disguised in another food that the patient will ingest. Typical total doses are the normal age adjusted single serving amounts, or 8 g to 10 g when freeze-dried or powdered foods are used. When freeze-dried or concentrated foods are used, the potential for alteration of labile allergens must be taken into consideration.

Standardized recipes for DBPCFC to milk, soy, cooked egg, raw whole egg, peanut, hazelnut, and wheat in their usual edible form, and validated by professional panelists in a food laboratory, have been published. Although the dosing and interval between doses can be established based upon the patient’s history, different schemes have been successfully used in different centers.

If subjective symptoms are encountered after a dose, options include waiting longer before administering the next dose, repeating the previous dose, or stopping the challenge. A food challenge is completed when the patient has an obvious reaction or a normal portion of the food has been ingested openly without symptoms.

The observation period following completion of the challenge depends upon several factors, including the immune mechanism involved, timing, severity and duration of previous reactions, whether the patient reacted and the severity of that reaction, in addition to the level of concern about biphasic anaphylaxis.

Usually patients are observed until they have been asymptomatic for a couple of hours after a reaction, or for about two hours after the last dose if the food was tolerated. Patients with nonIgE-mediated reactions, such as the food-protein induced enterocolitis syndrome or other delayed reactions, are observed longer.

The implications of the challenge results should be thoroughly reviewed with the patient and the patient’s family, and all questions thoroughly addressed. If the challenge is negative, including the challenged food regularly in the diet is encouraged.