MODEL RELEASED. Allergy test. Arm of a twenty five year old woman undergoing a skin scratch allergy test. This test is used to identify substances (allergens) that cause an allergic reaction in the patient. Weak solutions of various suspected allergens and controls are dropped on to the skin. Letters on the skin show the location of each allergen. The skin under each drop is then scratched with a needle to introduce the allergen under the skin. The skin is observed for a reaction. If an area of skin becomes swollen or red the patient is allergic to the substance at that location.