The diagnosis of food allergy is often based more on a careful evaluation of the patient than on laboratory tests. A person's history and symptoms may have characteristic features and lead to the recognition of food allergy when other conditions which cause similar symptoms have been excluded. To confirm the diagnosis, the patient will be put on a diet which excludes the suspected food or foods. If the symptoms disappear, the food will be reintroduced in order to confirm the diagnosis. Skin tests and a test called RAST (which detects circulating IgE antibodies) are sometimes helpful, especially in younger children. Unfortunately they are far from being totally accurate as diagnostic aids.
There are two major problems in identifying an allergenic food. Firstly, patients may react to not one but several different foods and secondly, the foods they eat may contain a wide range of allergenic ingredients. Bread, for example, may contain wheat, soya, yeast and milk - all of which are potential allergens. Ideally, to get around these problems an `exclusion diet' consisting only of water and a mixture of basic nutrients, should be used for diagnosis. The major drawback is that a diet like this could rapidly lead to malnutrition, especially in babies.
Many specialists prefer to use a so-called `oligoallergenic diet' (see below), which consists of well-defined unprocessed foods that generally produce few allergic reactions. Patients are put on this diet for a number of weeks and if their symptoms improve, they can gradually reintroduce foods one at a time, while watching for the reappearance of symptoms. As tell-tale reactions occur, the responsible food is recognised and eliminated. Since children tend to outgrow their food allergies, the offending food may be reintroduced each year to see whether the child has developed a tolerance to it. Very impressive results have been obtained in treating two common allergic conditions - atopic eczema and migraine - using oligoallergenic diets.
Oligoallergenic diets used to confirm the presence of food allergy
Stewed lamb Turkey
Rice and rice flour Potatoes and potato
Cabbage Carrots
Sweet potato Apples/pears
Milk-free margarine Olive oil
Milk substitute Soya milk
The 'double-blind challenge test'
Some authorities believe that the only way to diagnose food allergy is to use what is known as the 'double-blind challenge test'. In this test, dried foods in powder form are put into opaque capsules. Neither the doctor nor the patient must know what food is contained in the capsules. Patients are observed for up to 72 hours after taking the capsule and careful observation is made of any occurrence of symptoms following each 'challenge'. The major drawback of this method is that it bypasses the mouth, where many symptoms may begin. It is also not possible to use the test in babies and it is a very time-consuming procedure.
