Diagnosis of Food Allergy Not an Easy Task

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Category: Food Allergy Published on Thursday, 03 July 2008 Written by Yong Tsai, MD
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Most cases of food allergy occur when a person with particular genetic predisposition produces specific immunoglobulin E (allergy antibody) to the protein of a particular food.  When these IgE antibodies react with certain food, it triggers the release of histamine and other various chemicals from inflammatory cells, causing allergic symptoms which typically appear within minutes to two hours after a person has eaten the food to which he or she is allergic.  Moreover, there are some food allergies that is not mediated by IgE, such as food protein-induced gastrointestinal disorders in children.

Although all foods can cause an allergy, tree nuts, eggs, soy, milk, wheat, fish and shellfish account for 90% of all food-allergy reactions.  While, less common, additives such as dyes, sulfites, parabens, monosodium glutamate, butylated hydroxyanisole (BHA), butylated hydroxytolune (BHT) and nitrates can also cause allergic reactions.

Commonly used tests are blood the RAST (radio allergosorbent test) and the CAP ELISA (enzyme linked immunosorbent assay) as well as skin prick testing.   Both the blood tests and skin prick testing determine the presence of specific IgE antibody directed to particular foods.  Even if specific IgE is detected, a definite diagnosis cannot be simply based on positive specific IgE results, as up to 50% of patients with positive IgE tests do not in fact have allergy to a particular food.  Therefore, a positive prick skin test does not necessarily mean that the patient will experience an allergic reaction to a certain food.  The up side to these results is that negative tests are very reassuring that the food can be ingested safely without experiencing IgE-mediated food allergy.  Still, negative tests cannot exclude the possibility of non-IgE mediated food allergy.

The key to food allergy testing is that your allergist must interpret the test results while keeping your complete medical history in mind.   Sometimes, even when taking these factors (skin prick test, blood tests and medical history) into account, your allergist still cannot make a definite diagnosis.  Therefore he or she may recommend a food challenge: be asked to eat gradually increasing amounts of the suspected food under the physician's supervision while he/she observes you for any symptoms.  While, food challenges can offer definite answers, they can also carry the risk of serious reaction during challenge.   Sometimes, a food challenge is the only definite way to make a correct diagnosis; particularly if a food additive or a non-IgE mediated food allergy is present, as they will not produce positive skin tests or positive blood tests.

Diagnosing food allergy is not an easy task and it requires teamwork.  In addition to skin and blood tests, your detailed medical and dietary histories are crucial for a final diagnosis.

 

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