A Practical Diagnostic Approach to Food Allergies

Any abnormal reaction resulting from the ingestion of a food could be considered an adverse food reaction, and not necessarily a food allergy. In the evaluation of a patient with a history of an adverse reaction to food, one must consider the broad differential diagnosis before labelling the patient as ‘allergic’ to a foodstuff. The classification of adverse reactions to food is discussed, and different test modalities, with a practical diagnostic approach to food allergies, are outlined in this article. INTRODUCTION A abnormal reaction resulting from the ingestion of a food could be considered an adverse food reaction.1 In the evaluation of a patient with a history of an adverse reaction to food, one must consider the broad differential diagnosis before labelling the patient as ‘allergic’ to a foodstuff. Adverse food reactions can be broadly divided into toxic and non-toxic reactions (Figure 1).1 Toxic substances in food may affect any exposed individual, whereas nontoxic reactions are highly individual, and depend on genetic, epigenetic and environmental factors. Non-toxic reactions are grouped into i) immune mediated and ii) non-immune mediated reactions. The term food allergy is reserved for adverse reactions that involve the immune system. The term food intolerance is generally used in relation to non-immune mediated reactions. While the scientific basis of toxic and allergic (immune mediated) reactions to food is well established, the nonimmune mechanism of some types of food intolerance is less well defined.1,2 CLASSIFICATION OF FOOD ALLERGY Food allergy may be due to IgE-mediated, non IgE-mediated or a combination of IgEand non IgE-mediated reactions. Clinically it can involve the skin, gastrointestinal tract, respiratory tract and/or cardiovascular system. The prevalence of food allergy varies from 1 to 10% in children less than 5 years of age, dropping significantly in the adult population, as some allergies are outgrown.3,4 A. IgE-MEDIATED FOOD ALLERGY: The best characterised food allergies involve the IgEmediated immune mechanism. A failure to develop oral tolerance to food allergens (antigens) may lead to an excessive production of IgE-antibodies to the specific food. IgE-mediated allergies present typically within minutes to hours after ingestion of the specific food. Patients typically present with the following symptoms or conditions3: • Generalised: anaphylaxis, food dependent exerciseinduced anaphylaxis; • Cutaneous: urticaria, angioedema, flushing, acute contact urticaria; • Gastrointestinal: oral allergy syndrome, gastrointestinal anaphylaxis, colic, vomiting & diarrhoea; • Respiratory: acute rhino-conjunctivitis, allergic asthma. The prevalence of food hypersensitivity is the greatest during the first few years of life. In infancy and childhood the most common food allergens include egg, milk, fish, wheat, soya and peanut. In older children and adults the range of food allergens causative of hypersensitisation broadens to include seafood, tree nuts and fruits. Most children develop tolerance to food allergens by the age of 5-6 years, except in the majority cases of peanut, tree nut and seafood allergy.2 The diagnosis of an IgE-mediated allergy remains a clinical exercise dependent upon a clinical history, selective in vivo tests (skin prick tests) or in vitro measurement of food specific-IgE (sIgE), appropriate exclusion diet, and blinded provocation. To screen for food hypersensitivity against TABLE I: THE “BIG 8” FOOD ALLERGENS AND IMMUNOCAP® (RAST) FOOD GROUP TESTS INDIVIDUAL sIgE THE BIG 8 FOOD ALLERGENS Egg white, Milk, Fish, Wheat, Peanut, Soya, Shellfish, Tree nuts GROUP TESTS Mixed nuts (Fx1) Mix of Peanut, Hazel nut, Brazil nut, Almond, Coconut Mix of Cod fish, Shrimp, Blue mussel, Tuna, Salmon Mix of Wheat, Oat, Maize, Sesame seed, Buckwheat Mix of Egg white, Milk, Fish, Peanut, Soy and Wheat Mixed seafood (Fx2)