A skin prick test is the most common form of allergy testing. This test involves placing a small drop of an allergen extract on your skin. A needle is then used to prick the skin underneath the drop; this allows for a small amount of solution to enter just below the surface of the skin. After 15 minutes, any swelling or redness is measured and, depending on the size, is considered a positive reaction. An intradermal skin test is completed next. An intradermal wheal, or bleb, is injected directly under the top layer of skin. After 15 minutes any reactions are measured and classified as either positive or negative.