Chapter 2: Skin testing in allergy.

Skin tests are used in addition to a directed history and physical exam to exclude or confirm IgE-mediated diseases such as allergic rhinitis, asthma, and anaphylaxis to aeroallergens, foods, insect venoms, and certain drugs. There are two types of skin testing used in clinical practice. These include percutaneous testing (prick or puncture) and intracutaneous testing (intradermal). Prick testing involves introducing a needle into the upper layers of the skin through a drop of allergen extract and gently lifting the epidermis up. Other devices are available for prick testing. Intracutaneous (intradermal) testing involves injecting a small amount of allergen (0.01-0.02 mL) into the dermis. The release of preformed histamine from mast cells causes increased vascular permeability via smooth muscle contraction and development of a wheal; inflammatory mediators initiate a neural reflex causing vasodilatation, leading to erythema (the flare). Prick testing methods are the initial technique for detecting the presence of IgE. They may correlate better with clinical sensitivity and are more specific but less sensitive than intradermal testing. Sites of skin testing include the back and the volar aspect of the arm. Although the back is more reactive, the difference is minimal. By skin testing on the arm, the patient can witness the emergence and often sense the pruritus of the skin test reaction. Because more patients are sensitized (have IgE antibodies and positive skin test reactions) than have current symptoms, the diagnosis of allergy can be made only by correlating skin testing results with the presence of clinical symptoms.