Historical information that is important in determining the type of hypersensitivity reaction includes the route of exposure—for example, was the food ingested or inhaled, or was there just skin contact? The quantity of food ingested is also significant: Was the amount minute, small, or large? How was the food prepared? Was it raw or cooked? Has the patient had previous reactions to the food in its raw and cooked forms? Asking about spices or whether the food was mixed with other potential allergenic components is also necessary.
Information about whether or not someone else ate the same food and had a similar reaction would help to rule in or out infectious causes. A review of the patient’s current diet should include questioning as to whether the suspected allergen had been eaten again without reaction.
IgE-mediated allergy should be suspected when symptoms begin within minutes to hours of ingesting a food, occur repeatedly or in young children, or develop in an infant or young child who has been diagnosed with any of the following: moderate-to-severe atopic dermatitis, EoE, gastritis, enteritis/enterocolitis/enteropathy, or allergic proctitis.
Although the medical history and physical exam are the mainstays in establishing food allergy, the diagnosis can only be made when those findings are considered in combination with diagnostic testing. The most common lab studies include the SPT, IgE levels to specific antigens as determined by ImmunoCAP testing (Phadia, Uppsala, Sweden), total serum IgE, intradermal testing, and atopy patch testing. Intradermal testing, total serum IgE and atopy patch testing should not be used in routine evaluation of food allergy. SPT and serum measurements of specific IgE levels are recommended for identifying foods that may provoke allergic reactions, but they are not diagnostic.
For example, a patient who has a positive result on SPT to wheat and an elevated serum IgE to wheat may not necessarily have clinical symptoms that are reproducible. SPT may be available in a primary-care setting but is more likely performed by an allergist. The primary-care provider can order specific IgE levels, based on the patient’s clinical picture.
Patient history and physical examination findings are also crucial components of the diagnosis of non-IgE-mediated reactions, but the recommended diagnostic studies vary according to suspected illness. In patients thought to have EoE, the atopy patch test can be helpful in identifying the culprit foods. This should be done in combination with SPT and specific serum IgE tests.
The atopy patch test requires placement of a small quantity of each suspected allergen in an individual square chamber that is applied to the upper back. A patch is then adhered over the allergen and left in place for at least 48 hours. A positive reading includes erythema, infiltration of the surrounding skin, and occasionally papules. The dosing for atopy patches is not standardized, but the test seems to be useful in identifying late-phase clinical reactions.
Diagnostic studies are not commonly used to identify non-IgE-mediated FPIES. A medical history that includes the absence of symptoms after the suspected food is removed from the diet may be all that is necessary to diagnose FPIES.
An oral food challenge can be useful — unless there is a history of hypotensive episodes. Similarly, the diagnosis of food protein-induced allergic proctocolitis is typically based upon a medical history that includes resolution of symptoms after eliminating the causative food.
The gold standard
The oral food challenge is the gold standard for diagnosing food allergy.1,8 Because a severe reaction can occur with minute portions, any food suspected of triggering a reaction should be given in a setting by providers who can manage a potential anaphylactic event. Ideally, a double-blind, placebo-controlled food challenge is performed in a medical facility with appropriate supervision and medical treatments available.
Patients should never be instructed to go home and “give the food a try.” Whenever possible, neither the provider, the patient, nor the parent should know which food is being served. This eliminates any possibility that anticipation of a response may impact the results of the challenge. Open food challenges can be useful if a child is thought to be outgrowing an allergy.
Food allergy in children can be managed with dietary avoidance, nutritional counseling and education for the patient and family. Dietary avoidance is recommended for children with documented IgE- or non-IgE-mediated food allergy (i.e., positive results on the history, physical examination, laboratory findings, and oral food challenge). Dietary avoidance is not recommended for children without documented allergy, nor is it suggested in the management of atopic dermatitis or asthma as a means to decrease to prevent potential food allergy.
While there are different types of avoidance or elimination diets, the basic concept is to remove the suspected food for two to eight weeks. Symptoms will resolve if they are food-related and remain if they are not food-related. The diet must be monitored for nutritional adequacy; duration depends on the disease as well as on how well the patient’s nutritional needs are being met.
Limited elimination diets are typically employed when the provider suspects one of the more commonly allergenic foods (cow’s milk, egg, wheat, soy, fish) and the patient has either a positive result on SPT or a specific serum IgE level >0.35 ISAAC standardized units on ImmunoCAP testing.
Oligoantigenic elimination diets are useful when allergy to a large number of foods is suspected. This diet includes only those foods that have a low likelihood of allergenicity. The elemental diet, which can be most useful for infants, consists of a hypoallergenic (amino acid-based) formula with a few “safe” foods added, depending on the patient’s age. This is useful when allergy to a large number of foods is suspected or for infants who are not yet eating solid food. Compliance is an issue after infancy because children outside early infancy are less likely to transition easily to a hypoallergenic formula, and older children and their parents will have difficulty with a diet that includes very few foods.
Nutritional counseling consists of instruction on which specific foods to avoid and on proper reading of food labels and information on avoidance of those foods labeled as “this product may contain trace elements” of the identified allergen. The reason for avoiding such foods is that there is no way to know now much allergen might be present in the packaged food. There may be none, or there may be a large amount. This means the patient may be able to eat the food on one occasion, feel safe in eating it again, and then get a package with a much larger volume of allergen the next time. The potential for severe reaction may then be more likely.
Children with multiple food allergies should be referred to a dietitian or nutritionist to ensure that their dietary needs are being met. From a primary-care perspective, the child must have regular growth monitoring.
Because food allergy carries a risk of severe reaction and, potentially, death, patients and their families must have psychological support. Connecting families with support groups, counseling, or others with similar concerns is vital.
While no preventive measures are currently recommended, patients with food allergy must be prescribed epinephrine, typically in the form of an auto-injectable unit for ease of use. Epinephrine is indicated in the treatment of acute, systemic allergic reactions.
Antihistamines are used for managing nonsevere allergic reactions. Bronchodilators should be used if respiratory symptoms occur with exposure to the suspected food. Allergen-specific oral and sublingual immunotherapy is currently under study for inducing clinical desensitization but is not recommended for use in clinical practice.
Patients should be instructed to use an antihistamine if physical contact with an allergenic food causes cutaneous, ocular, or upper-respiratory-tract symptoms. Diphenhydramine is the antihistamine of choice and should be administered in liquid or chewable form for most rapid effect; the dose in children is 1-2 mg/kg. Patients who have asthma should be given a prescription for a rapid-onset bronchodilator as well as other asthma medications, depending on the severity of their disease.
Auto-injectable epinephrine must be administered promptly after the ingestion of an allergenic food. This is first-line treatment in all cases of suspected anaphylaxis. Delayed or improper use of the auto-injectable unit is a major cause of death.