Allergy testing is used to find out which substances produce an allergic reaction in a person. There are three common skin test methods for allergies. The first one, the skin puncture test, which consists in placing on the forearm or upper back a small amount of the substances that may be causing the symptoms on the skin. Once this is done, the skin is pricked so the allergen goes under the skin surface. 15 minutes later the reaction to each substance can be observed. The second test is the intradermal skin test, which involves injecting a small amount of allergen into the skin and observing if there is any reaction. It is most commonly used to detect allergy to a specific substance. Finally, the patch test, a method to diagnose the cause of skin reactions that occur once the substance touches the skin.