Dr. Amin Kanani (biography and disclosures)
Dr. Kanani has been a member of an Advisory Board: Pfizer, Sanofi, Merck, Novartis. He is involved in research grants and funding from industry: Novartis. He has participated in a clinical trial with Novartis. He has received honorarium from Pfizer, Sanofi, Merck, and Novartis. No conflict of interest related to the content of this article.
Frequently asked question I have noticed
Patients are often referred to determine if they have a food allergy. The prevalence of food allergy in Canada is 6.7%, however more than 20% on the population modifies their diet because of perceived food allergy. Some of these patients have had allergy testing that has no scientific validity or have had testing that was not interpreted correctly.
Data that answers this question
Approximately 85% of all food allergies are to peanut, tree nut, cow’s milk, egg, wheat, soy, sesame and seafood. These foods are classified by Health Canada as priority allergens.
A food allergy is a type of adverse reaction to food that involves an immunological mechanism. It can be IgE mediated or non-IgE mediated. Individuals diagnosed with an IgE mediated food allergy are at risk of anaphylaxis. IgE mediated reactions also involve oral allergy syndrome, eosinophilic esophagitis and atopic dermatitis. Oral allergy syndrome is a condition in which individuals with pollen allergy react to certain fruits, vegetables and tree nuts that share similar allergens. These cross reacting allergens are heat labile and therefore cooked version of the food can be tolerated. The symptoms are usually confined to the oral region (hence the name) and rarely progress to anaphylaxis. Eosinophilic esophagitis and atopic dermatitis are conditions that can involve a mix of IgE mediated and other immunological mechanisms, where foods may play a role in exacerbation. Celiac disease is an example of an immunological food reaction that is non-IgE mediated.
There are many non-immunological adverse food reactions that should not be referred to as food allergy. Examples include lactose intolerance, palpitations from caffeine, food triggers for migraine headaches. Some individuals simply make associations between foods and symptoms or a condition they are experiencing. It is incorrect to refer to these as food allergy.
This article will focus on the IgE-mediated food allergy and risk of anaphylaxis. Diagnosis is crucial for food allergy, as having that diagnosis implies an individual is at risk for a severe allergic reaction with sudden onset and carries a risk of death.
Patient history is important in determining if an individual has a food allergy. Allergic reactions occur fast after exposure to the food usually within seconds or minutes or up to 3 hours. Reactions also resolve fast, usually within hours to a day. Rare cases of protracted reactions can occur. To remember the symptoms of anaphylaxis, the acronym FAST can be used: Face: itching, redness, swelling, Airway: trouble breathing, swallowing, speaking, Stomach: pain, vomiting or diarrhea, Total: hives, rash, itching, swelling, weakness, paleness, sense of doom, loss of consciousness.
Diagnostic tests utilized are skin prick test and blood test for specific IgE to food protein. The gold standard test is a double blinded oral food challenge. Skin test or blood test cannot determine severity of the food allergy, rather the size of the wheal or the level of the food specific IgE helps determine probability of being allergic. A positive skin prick test does not mean food allergy. In atopic individuals, false positive reactions are not uncommon. The skin prick test has a sensitivity of 95% and specificity of 50%. A negative skin test is helpful in ruling out an allergy. A positive skin test only suggests an individual could be allergic and the size of the wheal can help determine probability of reacting. Allergy skin prick tests should be reported as millimeter size of the wheal. Studies suggest that a wheal size greater than a certain amount indicates a high probability of having a clinical reaction if exposed to that food. For example a skin test for peanut of 8 mm or greater has a positive predictive of 95%. The 1+ to 4+ scale for skin testing should no longer be used as it cannot provide this information. Blood test for specific IgE to food also determines probability of reacting. The specific IgE blood test has a low sensitivity and therefore cannot be used to rule out a food allergy. If the history is not clear and the tests are indeterminate then the gold standard test is an oral food challenge (many allergist conduct open challenges as blinded challenges are difficult to arrange). Once an individual has been diagnosed with a food allergy, risk for severe life-threatening reaction is always a possibility and carries significant implications: strict food avoidance, carrying epinephrine auto injector and seeking immediate medical attention. Skin testing or blood work conducted by a physician who does not interpret the test can lead to incorrect diagnosis of food allergy and confusion for the patient and other physicians, as well as stress, anxiety and unhealthy restrictive diets for the patient.
There is a philosophy to attribute foods as the cause of various symptoms or diseases (such as migraines, irritable bowel syndrome, arthritis, autism or attention deficit hyperactivity disorder). A variety of tests are sold for food allergies/sensitivities such as measuring IgG to food protein. IgG antibodies to food proteins are not pathological and are normally produced by all individuals particularly to foods recently eaten. Patients should be advised to disregard these tests as they have no scientific merit.
What I recommend (practice tip)
- History is the most useful tool in determining food allergy. Remember FAST.
- If a patient has eaten the food in question after the possible reaction and did not experience any symptoms, then they have conducted the gold standard test and should not be considered allergic to that food.
- If the patient’s history suggests an IgE mediated reaction to a food, then refer to an allergist to perform appropriate tests and provide useful interpretation of the results.
- Once a patient is diagnosed with a food allergy, they need to be educated on allergen avoidance, have an anaphylaxis management plan, carry an epinephrine auto injector and seek immediate medical attention if exposed.
- Educate patients about the lack of validity of certain food tests on the market (ex. IgG levels).
- Waserman S, Watson W: Food allergy. Allergy Asthma Clin Immunol 2011, 7(Suppl 1):S7 (View)
- Sicherer S, Sampson H. Food allergy: Epidemiology, pathogenesis, diagnosis, and treatment. J Allergy Clin Immunol 2014, 133: 291–307.e5. (View with UBC or CPSBC)
- Carr S, Chan E, Lavine E, Moote W. CSACI Position statement on the testing of food-specific IgG. Allergy, Asthma & Clin Immunol 2012, 8:12 (View)
- Bird JA et al., Food allergen panel testing often results in misdiagnosis of food allergy. J Pediatr 2014 Sep 10 (View with UBC or CPSBC)