Allergy patch test. Young woman with four allergy test strips on her back. On each test strip are ten patches, each of which is used to establish sensitivity to certain allergens. This is called a patch test. Small amounts of a variety of common allergens, such as pet hair, pollen and house dust, are placed onto various patches. The patches are then stuck onto the patient's skin, in some cases after the skin is pricked. Allergic reactions cause the skin around the relevant patch to become inflamed, denoting the allergen to which the patient is sensitive. By knowing the allergy, the patient can alter their diet or behaviour to avoid the allergen.