Medical Health Encyclopedia

Allergy testing

While this strategy is useful and practical for mild allergic reactions, it must be undertaken carefully in individuals with suspected severe reactions to foods. In this instance, blood tests may be a safer first approach, as described below. Of note, skin testing is almost never performed to detect food allergies for the same reason of the higher risk of causing a severe allergic reaction.

The RAST is a laboratory test performed on blood (see venipuncture ). It tests for the amount of specific IgE antibodies in the blood, which are present if there is a true allergic reaction.

Other tests include immunoglobulin measurements (see serum immunoglobulin electrophoresis) and the blood cell differential or absolute eosinophil count (increased eosinophils can indicate the presence of allergy). Provocation (challenge) testing includes exposure to the suspected allergen -- for example, in the diet or by inhaling the suspected allergen -- under controlled circumstances. This type of test may provoke severe allergic reactions. Challenge testing should only be done by a doctor.

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How to prepare for the test:

Before any allergy testing, the health care provider will ask for a very detailed medical history. This may include questions about such things as illnesses, emotional and social conditions, work, entertainment, lifestyle, foods, and eating habits.

If skin testing will be performed, it is essential that you not take antihistamines prior to the test. This may lead to a false-negative result, falsely reassuring you that a substance is unlikely to cause a severe allergic reaction. Your doctor will tell you which medicines to avoid and when to stop taking them before the test.

How the test will feel:

Skin tests may cause very mild discomfort when the skin is scratched or pricked. Itching may accompany a positive reaction to the allergen.