Allergy tests usually consist of skin tests or blood tests. An allergy test determines the allergen, i.e., the substance that triggers an allergic response in that person. Skin tests can be conducted rapidly, and these are more reliable and cheaper than blood tests. There are different types of skin tests. A small amount of the allergen may be applied on unbroken skin to see if it evokes a reaction. A skin prick test involves piercing the skin with a needle and then placing a drop of the possible allergen in solution form over the punctured area. If this area of the skin gets inflamed, it means that the person is allergic to that particular allergen.
The intradermal test has a small amount of the allergen in solution being injected into the skin. This test is performed when a substance does not evoke an allergic reaction in the skin prick test, but is still suspected to be an allergen. In the skin patch test, a pad containing the allergen in solution is taped on the skin for 24-72 hours. It is used to detect a type of skin allergy known as contact dermatitis.
Blood tests for allergies usually look for a specific variety of antibodies. These are less sensitive as compared to skin tests. People who are unable to undergo skin tests go in for the blood test. The Radioallergosorbent test (RAST) is the most common blood test, which looks for immunoglobulin E (IgE) antibodies in the blood. These antibodies are produced in response to specific allergens.
Allergy tests are basically performed to identify respiratory allergens (like trees, weeds, shrubs, dust, mold, feathers, and pet dander), food allergens (such as milk, fish, and peanuts), allergies triggered by medicines or insect venom, and also substances that evoke hypersensitive responses from the skin when they come in contact with it.
The back part of the arm is usually the site for the application or skin prick test. One drop of each of the suspected allergens is placed on the skin and the area under the drop is pricked. After 10-15 minutes, the skin is observed for any inflammatory changes. If this test is negative, the intradermal test is performed. Similarly, if the intradermal test is negative, the patch test is performed. The blood test for allergies is performed like any other blood test. The blood sample is collected and then tested for the presence of antibodies.
The result of an allergy test decides the line of treatment for a particular type of allergy. If a person is allergic to something and his skin is exposed to that allergen, weals develop in that area of the skin, which resemble mosquito bites. In allergic patients, the level of IgE may be 4 times higher than in normal individuals. There are some risks involved even when testing for allergies. A severe allergic reaction, known as anaphylaxis, may occur and even threaten the patient's life. In the skin patch test, the patient may actually develop a patch of contact dermatitis in the area of the skin being tested and will have to be treated for it by a dermatologist.
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