Allergic Skin Tests

Allergic skin tests, including skin prick tests (SPT), are executed by a trained nurse in the allergist's office. Before the tests, antihistamines anf other histamine-blocking medications should be discontinued according to their half-life of elimination. When performing a prick or scratch test, a tiny drop of a possible allergen is pricked or scratched onto a certain area of the epidermis on the volar forearm, whereas a small amount of the suspect allergen is injected by a thin needle (intradermal test, more sensitive for some allergen sensitization) inside a particular area of the epidermis. A prick/scratch test is assessed after 20 minutes taking into consideration the diameter of the wheal. An intradermal test is evaluated by diameter of the papule after 20 minutes (early phase), 4-6 hours (late phase), and in 48-72 hours (type IV hypersensitivity). In both cases, histamine is used as a positive control, whereas saline is exploited as a negative control to assess the wheal reaction of the skin before testing.