Allergy test. Arm of a patient undergoing a skin prick allergy test. This test is used to identify substances (allergens) that cause an allergic reaction in the patient. Small amounts of suspected allergy-causing substances are placed on the skin (as seen here), then the skin is pricked with the tip of a scalpel blade in order to introduce them under its surface. The skin is then observed for signs of a reaction, which usually develop after several minutes. These include redness and swelling.