Food Challenges: Why Bother?
By Dan Atkins, MD
Food challenges play a vital role in the evaluation and management of patients with histories suggestive of food allergy. The necessity of food challenges is supported by studies revealing that more than 50% of patients presenting with a history of an adverse reaction to a food fail to react during blinded challenges to the suspected food. There are a number of reasonable explanations for this.
Sometimes the wrong food is suspected as the cause of symptoms. For example, a child reacting in a restaurant to a French fry cooked in peanut oil might be suspected of having a reaction to peanut, when the actual cause was contamination of the French fry with fish protein from fish fried in the same oil.
Sometimes the reaction is caused by a nonfood contaminant as exemplified by reactions to latex proteins deposited on foods by food handlers wearing latex gloves or reactions to dust mites in mite-contaminated baked goods.
Sometimes the reactions are not related to food ingestion at all, but are precipitated by medications, toxins, parasites, allergen exposures by inhalation or contact, viral illness, exercise or panic, to list just a few potential causes.
Skin testing can identify foods that may have caused symptoms, but the positive predictive accuracy of a properly performed food prick skin test is only about 50%. Thus, there are patients with reasonable histories of food allergy and a positive skin test to the suspected food who do not react during food challenge.
The CAP System FEIA (CAP FEIA) is a quantitative antibody fluorescent-enzyme immunoassay that measures the amount of circulating allergen-specific IgE in the serum in kilounits of antibody (allergen-specific IgE) per liter (kUA/L). For several foods, such as milk, egg, peanut, soy, wheat and fish, threshold CAP FEIA levels have been calculated. Patients with CAP FEIA values higher than the threshold values have a 95% likelihood of reacting after ingestion of the food. However, individual CAP FEIA results may fall in the gray zone below the threshold or threshold values may not have been established for the suspected food. As a result of these and other nuances, food challenges are often necessary to document the association between the ingestion of the suspected food and the onset of symptoms.
Performing food challenges in patients with suspected food allergies is often anxiety producing for patients and their families. At first glance having someone eat a food that might make him or her ill seems contrary to that basic premise of medicine (and parenthood) "first do no harm." However, there are a number of situations where the information to be gained is worth the risk. It is also important to remember that in experienced hands with appropriate attention to detail and sound clinical judgment, food challenges can be safely performed. Experience over three decades at National Jewish proves that food challenges can be administered safely.
Food challenges are performed to address a variety of clinical questions. Sometimes the food responsible for the reaction is not apparent from the history or after attempts to document sensitization. For example, the patient may have a positive skin test to several foods ingested before a reaction and food challenges may be necessary to determine which, if any, of the suspected foods is the culprit. Determining which food actually caused the reaction is necessary to aid in preventing future reactions and to avoid needlessly eliminating foods from the diet.
Documenting the degree of sensitivity is another reason for performing food challenges. Some patients or their families become concerned that exposure to even miniscule amounts of a food might cause a reaction. Others are worried that any exposure would result in a life-threatening reaction. These concerns occasionally inhibit participation in normal activities and can lead to social isolation. Although some patients are indeed exquisitely sensitive, others find that a larger exposure to the food than was expected can be tolerated without a severe reaction. This can be liberating for patients who have markedly curtailed activities out of concern about the possibility of exquisite sensitivity. Some patients, despite large positive skin tests and histories suggestive of contact reactions, do not develop systemic symptoms after the ingestion of significant amounts of the suspected food. Alternatively, some patients are found to be more sensitive than was previously suspected and the importance of strict avoidance as well as being thoroughly prepared to treat severe reactions is reinforced.
Some food challenges are performed to prove that a food is not the cause of symptoms. An example is the patient who has been inaccurately labeled as allergic to one or more foods despite an unconvincing history or suspicious skin test results. A food challenge is indicated to find out if ingestion of the food causes symptoms.
Some patients outgrow their food allergies. The majority of infants and young children allergic to milk, egg or soy outgrow their food allergies by their third birthday. Studies over the past few years have even suggested that approximately 20% of children with allergic reactions to peanut in the first years of life outgrow their sensitivity (see ARIA Journal Club). CAP FEIA results can help in making a decision about when to challenge these children. Skin tests often remain positive even when the food allergy has been outgrown. The careful performance of food challenges can safely document when the food can be returned to the diet or if continued avoidance is necessary.
Food challenges should be performed in a medical setting with the necessary medications and equipment as well as personnel experienced in the treatment of anaphylaxis. Decisions about who should be challenged are reached only after a thorough evaluation and discussion of the risks and benefits with the patient or his or her family. However, few procedures in medicine answer a posed clinical question as directly as a properly performed food challenge and the information obtained can be life altering.
