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Allergic diseases include atopic or immunoglobulin E (IgE)–mediated disorders that classically involve allergic rhinoconjunctivitis, allergic asthma, food and mediation allergy, and anaphylaxis. Although not mediated by IgE, certain diseases are often grouped with allergic diseases and include eosinophilic diseases, mast cell disease, nonallergic rhinitis, chronic rhinosinusitis, and primary immunodeficiency.
Standard allergy testing relies on identifying IgE antibody specific for the allergen in question. Two classic methods of doing this are the immediate wheal-and-flare skin prick tests (in which a small amount of antigen is introduced into the skin and the site is evaluated after 15 minutes for the presence of an immediate wheal-and-flare reaction) and in vitro (blood) testing.
Methods of allergy testing that do not have a clear scientific basis include cytotoxic testing, provocation-neutralization testing or treatment, and yeast allergy testing.
Patch Tests and Skin Prick Tests
Patch testing of skin is not the same as immediate wheal-andflare skin prick testing. Patch testing is used to investigate only contact dermatitis, a type IV hypersensitivity skin reaction. Patch tests require 72 to 96 hours for complete evaluation. Many substances cause contact dermatitis. Common contact sensitivities include those to nickel, formaldehyde, fragrances, and latex.
Skin prick testing, by comparison, identifies inhalant allergens that cause respiratory symptoms, such as allergic rhinitis and asthma. These allergens include dust mites, cats, dogs, cockroaches, molds, and pollen of trees, grass, and weeds. Food allergy is also assessed with skin prick testing.
Skin prick testing and intradermal testing involve introducing allergen into the skin layers below the external keratin layer. Intradermal testing, the deeper technique, is used to evaluate allergy to stinging insect venoms, penicillin, and other medications. Intradermal tests are preceded by skin prick tests.
Drugs with antihistamine properties, such as histamine1 (H1) receptor antagonists, and many anticholinergic and tricyclic antidepressant drugs can suppress the immediate response to allergy skin tests. Use of nonsedating antihistamines should be discontinued 5 days before skin testing. Histamine2 (H2) receptor antagonists have a small suppressive effect. High-dose corticosteroid treatment can suppress the delayed-type hypersensitivity and the immediate response.
In Vitro Allergy Tests
In vitro (blood) allergy testing initially involves chemically coupling allergen protein molecules to a solid-phase substance and ultimately measuring the patient’s IgE specific to the allergen through radiolabeling, colorimetry, or other markers.
This test identifies the presence of allergen-specific IgE antibody in the same way that the allergen skin test does. Generally, in vitro allergy testing is not as sensitive as skin testing and has limitations because of the potential for chemical modification of the allergen protein while it is being coupled to the solid phase. Generally, in vitro allergy testing is more expensive than allergen skin tests and has no advantage in routine clinical practice. The test may be useful clinically for patients who have been taking antihistamines and are unable to discontinue their use or for patients who have primary cutaneous diseases that make allergen skin testing impractical or inaccurate (eg, severe atopic eczema with most of the skin involved in a flare).