Skin allergy testing is a method for the medical diagnosis of allergies that attempts to provoke a small, controlled, allergic response.
A tiny amount of an allergen is introduced into a patient's skin by various means. In the prick test, the skin is pricked with a needle containing a small amount of the allergen. If there is a large enough response with a significant increase in the size of the bump, or wheal produced by the injection, the patient is likely to be allergic to that particular allergen.
Another frequently used test is the intradermal test, whereby one or more injections of a diluted amount of the allergen in question is injected with a very small needle and syringe just under the skin surface, and the resulting skin reactivity measured a few minutes later. This test is more sensitive than the prick test, and may be used as the primary form of skin testing, or be done to confirm or negate a borderline prick test, or to confirm a suspected allergen identified by RAST or invitro testing.
No matter what skin test is selected, in all cases where the test is positive, the skin wheal will become raised and firm, and the surrounding skin is red and itchy. The results are recorded, with larger wheals indicating that the subject is more sensitive to that particular allergen. A skin test may be falsely negative when the patient has taken an antihistamine within a few days of testing, or in some medical conditions such as low thyroid. In those cases, there are blood tests, commonly referred to as in vitro or RAST tests, which measure a specific antibody in the blood involved with airborne and some food allergens, which may be very useful for testing purposes.