subject: Patch Tests and Prick (Cutaneous) Tests [print this page] Standard allergy testing relies on identifying the IgE antibody specificfor the allergen in question. Two classic methods of doing this are theimmediate wheal-and-flare skin prick tests (a small amount of antigenis introduced into the skin and evaluated at 15 minutes for the presenceof an immediate wheal-and-flare reaction) and in vitro testing.Allergy testing that does not have a clear scientific basis includescytotoxic testing, provocation-neutralization testing or treatment,and "yeast allergy" testing.Patch Tests and Prick (Cutaneous) TestsMany seem confused about the concept of patch testing of skin asopposed to immediate wheal-and-flare skin testing.
Patch testing isused only to investigate contact dermatitis, a type IV hypersensitivityreaction. Patch tests require about 96 hours for complete evaluation(similar to tuberculin skin reactivity, which requires 72 hours). Mostsubstances that cause contact dermatitis are small organic moleculesthat can penetrate various barriers inherent in the skin surface. Themechanisms of hypersensitivity postulated to explain these reactions usuallyinvolve the formation of haptens of endogenous dermal proteins.Inhalant allergens, in comparison, generally are sizable intactproteins in which each molecule can be multivalent with respect toIgE binding. These molecules penetrate the skin poorly and areseldom involved in cutaneous type IV hypersensitivity reactions.
They cause respiratory symptoms, such as allergic rhinitis and asthma,and are identified by skin prick testing. Their sources include dustmites, cats, dogs, cockroaches, molds, and tree, grass, and weed pollens
. Patch testing is used to investigate contact dermatitis
. Skin prick (immediate) testing is used to investigate respiratoryallergy to airborne allergens.Prick, scratch, and intradermal testing involve introducing allergento the skin layers below the external keratin layer. The deeper techniquesare more sensitive but less specific. With the deeper, intradermaltests, allergen is introduced closer to responding cells andat higher doses. Allergen skin tests performed by the prick techniqueadequately identify patients who have important clinical sensitivitieswithout identifying a large number of those who have minimal levelsof IgE antibody and no clinical sensitivity. Intradermal testing is usedin selected cases, including evaluating allergy to stinging insect venomsand to penicillin.
Drugs with antihistamine properties, such asH1 receptor antagonists, and many anticholinergic and tricyclicantidepressant drugs can suppress immediate allergy skin test responses.The H2 receptor antagonists have a small suppressive effect.Corticosteroids can suppress the delayed-type hypersensitivity responsebut not the immediate response. Intradermal skin tests are more sensitive but less specific thanprick skin tests. Intradermal skin testing is used to investigate allergy to insectvenoms and penicillin.In Vitro Allergy TestingIn vitro allergy testing initially involves chemically coupling allergenprotein molecules to a solid-phase substance.
The test is then conductedby incubating serum (from the patient) that may containIgE antibody specific for the allergen that has been immobilized tothe membrane for a standard time. The solid phase is then washedfree of nonbinding materials from the serum and incubated in asecond solution containing a reagent (eg, radiolabeled anti-IgE antibody).The various wells are counted, and the radioactivity is correlateddirectly with the preparation of a standard curve in whichknown amounts of allergen-specific IgE antibody were incubated witha set of standard preparations of a solid phase.
In vitro allergy testinguses the principles of radioimmunoassay or chromogen activation.It is important to understand that this test only identifies thepresence of allergen-specific IgE antibody in the same way that theallergen skin test does. Generally, in vitro allergy testing is not assensitive as any form of skin testing and has some limitations becauseof the potential for chemical modification of the allergen proteinwhile it is being coupled to the solid phase by means of covalentreaction. Generally, it is more expensive than allergen skin tests and hasno advantage in routine clinical work. In vitro allergy testing maybe useful clinically for patients who have been taking antihistaminesand are unable to discontinue their use or for patients who have primarycutaneous diseases that make allergen skin testing impracticalor inaccurate (eg, severe atopic eczema with most of the skin involvedin a flare).
