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Reagents used in tests performed on the skin to identify substances that produce an allergic reaction in a patient (i.e., allergens). Typically, a small amount of the suspected allergy substance (i.e., the allergen or \'test reagent\') is placed on the skin (usually on the forearm or back) and introduced under the skin by scratching or pricking the skin with a sterile needle. The skin is observed for about 20 minutes; the test is considered positive (probable allergy) if redness or, more importantly, swelling is present. These reagents may be also injected under the skin (intracutaneous), using a small syringe, or applied to the skin, usually mixed with a nonallergenic material in aluminum discs that are typically placed on the upper back and covered with tape for longer periods (typically 48 hours) to determine allergic contact dermatitis (i.e., patch testing). Skin test reagents are used mainly to determine specific substances that cause respiratory (e.g., dust, pollen) and food (e.g., seafood) allergies, but they are also used to test for allergies to drugs (e.g., penicillin) or to products (e.g., latex).
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