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In the event of an allergic reaction, a range of tests are available to determine which substance is the primary cause. Various combinations of skin, blood or challenge tests may be used, depending on symptoms and the patient’s medical history.
Skin tests are often the first choice due to their convenience and speed. They are used to screen potential allergens (allergy-causing substances) by applying them to the skin and observing whether they trigger a reaction.
A skin prick test is normally used to detect allergies that cause an immediate reaction on contact with the skin, such as hay fever or food allergies. Drops of a solution containing an allergen (such as animal hair or pollen) are applied to different marked areas of the skin. The skin is then pricked at each site to allow the allergen to enter the skin. For food allergies, the lancet is dipped in the food before pricking the skin.
An allergic reaction is indicated by skin reddening or a small bump (wheal) at the marked site.
An intradermal test is similar to the skin prick test, except that the allergen solution is injected directly into the skin. The primary advantage of an intradermal injection is that it can detect weaker allergic reactions.
The disadvantages, however, are the unpleasantness of the injection and the stronger allergic reactions that it can trigger. Therefore, intradermal tests are usually only performed when the response to a skin prick test is insufficient.
A skin scratch test is generally used if the skin prick test is inconclusive. A small section of skin is removed so that the allergen can be rubbed over the exposed flesh. Substances are able to penetrate deeper layers of tissue than with the skin prick test and, therefore, trigger stronger reactions. The skin scrape test is similar but removes only the top layer of skin.
These tests are less reliable than the skin prick test as they cannot control the amount of allergen applied and are more likely to trigger non-allergic skin irritation.
To reduce the risk of strong reactions, the substance can be rubbed on without breaking the skin. Reactions take longer to detect than the skin prick test, evidenced as red, bumpy skin after about 20 minutes if allergy occurs.
The patch test is used to screen substances if allergic symptoms are expected to be observed between one and three days after exposure to the allergen.
Patches containing different allergens are applied to a person’s back and left in place for 48 hours. The patches are then removed, and the skin examined for signs of a hypersensitive reaction. A second assessment is performed after 96 hours, and sometimes additionally after 7 to 10 days in some people.
Triggers are often single substances, found in a variety of materials, such as medications (e.g. lidocaine), cosmetics (preservatives or fragrances), jewelry (nickel or cobalt) or gloves (latex). If a contact allergy occurs, the skin swells, turns red, itches and small blisters may form.