MODEL RELEASED. Allergy test. Medical worker applying a solution to the 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. Weak solutions of various suspected allergens are dropped on to the skin. The skin under each drop is then scratched or pricked with a needle to introduce the allergen under the skin. If an area of skin becomes swollen or red the patient is allergic to the substance at that location.