A patch test is a method used to determine if a specific substance, such as a cosmetic or medication, causes an allergic reaction on the skin. During the test, a small amount of the substance is applied to a patch, which is then placed on a person's skin, usually on the forearm. The area is monitored for 48 hours to check for any signs of irritation or allergic response, such as redness or swelling. Patch tests are commonly used in dermatology to identify allergens in products like fragrances, preservatives, and dyes. If a reaction occurs, it helps healthcare providers recommend safer alternatives and manage conditions like contact dermatitis.