In skin-prick tests, extracts of a number of allergens are placed on the skin, usually on the inner skin of the forearm. The skin is then pricked or scratched, allowing the allergen to enter the skin. The skin is marked with a pen to show the site of each allergen; the individual being tested is not told what is being tested. If you are sensitive to the allergen, there will be a positive reaction, usually starting as an intense itching within a couple of minutes. Within 10-15 minutes, a raised blister-like weal will form, surrounded by a flare – an area of red itchiness around the weal. There are no side-effects or further symptoms.
The skin-prick test is most reliable at confirming allergy to things you inhale, such as house dust mites, moulds or animal hair, but it can be used to confirm sensitivity to foods or insect stings. It can, however, be misleading. You can have a positive result to a skin-prick test without actually having any symptoms. You can also get a false negative reaction to something to which you are clearly allergic, particularly with foods. As a result, most doctors use skin-prick tests as an aid to confirm a case history, not as a stand-alone test.
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