Thursday 10 December 2009, 11:58

Allergy skin tests

What are allergy skin tests and what are they used for?

Allergy skin tests

Allergy skin tests are used to: 1) confirm whether symptoms and signs such as skin rashes, sneezing or wheezing are due to allergy, 2) identify the specific allergens that trigger allergic reactions and, 3) help the care provider develop an appropriate and effective allergy treatment plan.


Skin testing is widely used to diagnose allergic conditions such as hay fever, dermatitis, food allergies, allergic asthma and allergy to pharmacological substances. It is based on the exposure of the skin to a variety of allergens (allergy-causing substances), followed by the observation of the presence or absence of signs of a local allergic reaction.


Although allergy skin tests are quite safe for most people, they are not recommended in some cases, such as in patients taking medications that can interfere with test results (e.g. antihistamines, tricyclic antidepressants and heartburn medications), in those with severe skin diseases (e.g. diffuse eczema, psoriasis) and in patients highly sensitive to suspected allergens, in order to avoid a severe immediate allergic reaction (anaphylaxis). In the above cases, blood tests are recommended.


Allergy skin tests can be divided into three main categories (percutaneous, intracutaneous or epicutaneous test), depending on how deeply allergens are applied or injected into the skin. The percutaneous test (prick-puncture test), in which a tiny amount of allergen is scratched across or lightly pricked into the skin, is usually performed to assess allergy to pollen, pet dander, mold, dust mites and foods. The intracutaneous test (intradermal test), in which a small amount of the allergen is injected into the skin of the arm, is usually performed to identify allergies to penicillin or insect venom. Finally, in the epicutaneous test (patch test), the allergen is applied to a patch, which is then placed directly on the skin surface. This test is usually performed to identify substances that cause contact dermatitis, such as latex, medications, fragrances, hair dyes and metals.


While prick-puncture and intradermal tests detect immediate allergic reactions, which develop within about 15 minutes of exposure to an allergen, patch test detects delayed allergic reactions, which develop over a period of several days. Regardless of the test method used, a positive result will be indicated by the development of a raised, red, itchy bump on the area of the skin exposed to the allergen.


It is important to stress that allergy skin tests are not always reliable. A negative result does not necessarily mean that the tested person is not allergic to the tested allergen. On the other hand a person may react positively to an allergen during a test, but not react to it in everyday life.


On the whole, skin tests are most reliable for assessing allergic reactions to airborne substances (pollen, dust mites, pet dander), while their reliability in assessing food-allergies is not always clear and additional tests or procedures are often necessary.


By Chiara De Carli

Category: Allergy

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