Allergy testing is usually done by testing the skin or the blood for the presence of a reaction to an allergen. There are three principal types of allergy testing used in modern medicine: skin testing, elimination testing, and radioallergosorbent testing (RAST). Tests done on the skin are more common than lab work on blood, because skin testing is less costly and somewhat more reliable than a blood test. Other testing methods besides those above exist as well, such as the astonishingly simple, yet effective methods used by naturopathic physicians.
Within the category of skin testing, there are three types of tests commonly done. The first of these is called a skin patch test, in which a small pad, moistened with a solution of the allergen in question is taped against the skin for a period of 24 to 48 hours. If the patient is allergic to the substance, an allergic reaction known as contact dermatitis will appear. Allergy testing by this method is fairly reliable, but other, faster methods also exist.
The skin prick test is done by placing a few drops of an allergen in solution onto the person's skin. A needle is then used to scratch or prick the skin enough so that the allergen can enter it. If the person is allergic, a raised, red area will develop on the skin. An intradermal test works in a somewhat similar way. In this method of allergy testing, the allergen solution is injected in small quantities into the skin.
More than one allergy can be tested for at the same time in this way. The one possible drawback to the intradermal method is that is is significantly more sensitive than a skin prick test, and can at times give a false positive. This means that the test results will look as if there is an allergy to that substance, when there is actually not.
When the allergies in question are food allergies, the elimination method of allergy testing is often used. As part of the elimination method, the suspected allergens are eliminated from the person's diet for a period of weeks, and are then reintroduced one by one. The food allergy is detected if an allergic reaction occurs shortly after a certain food is reintroduced. The elimination method can only be used to test for non-life threatening allergies.
If a patient is suffering severe reactions to an unknown allergen, blood testing is usually the preferred method. The RAST test is one of the types of blood testing used in this case. Blood tests measure the levels of antibodies present in the blood, which are produced as part of an allergic reaction. Only a few RAST tests are available, compared to the number of allergies that can be detected through skin testing.