If you suspect you might be allergic to something, your doctor will probably refer you to an allergist who can test to determine whether you have allergies, and if so, to what. One of the most common ways to test for allergies is through an allergy skin test.
There are three types of skin tests in which extracts of allergens are applied to your skin; none of them are very painful. The first (and most common) is called a puncture, prick or scratch test. In it, drops of the suspected allergens are put on your forearm or upper back, and then your skin is lightly scratched so that the allergen can seep in a bit. This test is done to detect immediate reactions to allergens like dust mites, mold, dander, pollen and various foods. If you're allergic to any of the substances the doctor is testing for, a little bump will develop on the site of the prick within 15 minutes. It will go away after about 30 minutes. In an intradermal test, allergen extracts are injected into the skin of your arm. This test is most common when the doctor suspects you're allergic to insect venom or to penicillin. A patch test uses stick-on patches instead of needles to test for allergies. It's most commonly used to check for allergies that cause a response on contact, like latex, fragrances, dyes, metals and resins.
When the doctor checks for allergens, he also will do a control test in which you're checked for a reaction to histamine. Almost everybody reacts to histamine; he's checking to make sure your body knows how to respond to an allergy test. At the same time, he'll test for a reaction to glycerin or saline. If you don't, he'll know your skin knows when not to react. If you do react, your skin might be too sensitive for a reliable allergy test.