Author: Agnes LEUNG
Assistant Professor, Department of Paediatrics, CUHK
Food allergy is categorized into three types based on the nature of the immunological response: IgE-mediated (e.g. anaphylaxis), non-IgE-mediated (also called cell mediated, e.g. food protein induced enterocolitis syndrome), and mixed IgE- and non-IgE-mediated (e.g. flare of atopic dermatitis). As doctors, we are particularly concerned when a child has an IgE-mediated food allergy. IgE-mediated food reactions occur when a food allergen binds to allergen-specific IgE on mast cells and basophils, resulting in the release of numerous mediators, including histamine. The responses happen quickly (within two hours) and might potentially evolve into anaphylaxis with severe respiratory, or cardiovascular symptoms. Symptoms resolve within hours and are reproducible when the culprit food is consumed again. Food allergen avoidance is currently used to manage food allergy; hence it is critical that particular foods be identified through allergy tests in order to avoid unnecessary food avoidance and the psychological burden on families. Individual foods should be identified in those who are at risk of anaphylaxis owing to food allergies. Adrenaline (epinephrine) injectors are often employed to treat life-threatening anaphylaxis caused by inadvertent exposure to known food allergens.
It is critical to seek medical attention because the diagnostic process always begins with the patient’s medical history, which is a critical step in identifying food allergy as a possible source of symptoms, providing information indicating whether the response is IgE-mediated, and identifying the most likely allergen(s) trigger. After identifying the suspected food allergen, the first-line diagnostic testing comprises of skin testing or measuring serum food-specific IgE levels. Skin testing involves the introduction of allergens into the epidermis through extracts and a device that scratches or punctures the skin. Sensitization is indicated by the appearance of a wheal and flare. Serum food-specific IgE testing reveals IgE antibodies to specific foods (i.e., sensitization). Both types of testing are quite sensitive in detecting IgE-mediated food allergies. It is reported that skin testing has a sensitivity of more than 90% and serum food-specific IgE measurement has a sensitivity of 70%-90%. Both results are useful tools to predict the likelihood of allergy, but they do not correlate well the reaction severity, i.e. the probability of anaphylaxis. Skin testing offers the advantages of being less expensive and providing rapid results. Serum food-specific IgE testing is becoming more widely available in primary care, which may increase the capacity to address variations in findings over time. However, it is important to remember that allergy tests should only be performed in the context of a relevant clinical history. Sensitization may not always imply clinical allergy, which might lead to unnecessary food avoidance.
Food “sensitivities” are allegedly detectable via food-specific immunoglobulin G (IgG) testing. Food-specific IgG test, however, has not been validated or researched. In fact, food-specific IgG merely indicates the presence of food exposure. International and local allergy societies respectively issued statements strongly opposing the use of IgG testing to identify an adverse food reaction. Other procedures that are not recommended are applied kinesiology and electrodermal testing.
There is no standardized step-by-step method for diagnosing food allergies. To assess IgE-mediated food allergies, skin testing, serum food-specific IgE testing, or both may be utilized initially. They should not, however, be used as the sole method of diagnosis since the clinical history and the completion of medically supervised oral food challenges, are important additional components of the diagnostic regimen. In some circumstances, other diagnostic modalities, such as component resolved diagnostics and the basophil activation test, can be useful supplemental diagnostic tools to help identify the presence of a food allergy.