To maximize the positive predictive value of allergy testing, a positive test result must be correlated with the history. Patients selected for testing include those with moderate to severe disease, those who are potential candidates for allergen immunotherapy, and those with strong predisposing factors for atopic diatheses, eg, a strong family history of atopy or ongoing exposure to potential sources of allergen. Since the development of rhinitis precedes the presentation of asthma in over half of cases, early intervention may decrease the risk of more severe clinical illness. The type of immune response must be consistent with the nature of the disease. For example, IgE antibody causes allergic rhinitis but not allergic contact dermatitis. IgE antibodies are detected by in vivo (skin tests) or in vitro methods.
Epicutaneous or cutaneous allergen testing produces a localized pruritic wheal (induration) and flare (erythema) that is maximal at 15–20 minutes. Such testing is used most commonly in the diagnosis of allergic respiratory disease (rhinitis and asthma) and suspected hypersensitivity to foods, drugs, and hymenoptera venom. Allergen extracts are available for pollens, fungi, animal danders, and dust mites and are selected appropriately for the patient's geographic area.
In vivo skin testing is preferred to in vitro methods because it detects the presence of IgE antibody in tissue and shows its biologic activity. Skin testing is generally more sensitive, more specific, more rapid, and less expensive than in vitro immunoassays. Any drug with antihistaminic effects (eg, H1-antagonists, tricyclic antidepressants, phenothiazines) must be withdrawn prior to skin testing. Appropriate controls with a negative diluent and a positive histamine response are mandatory for valid results and accurate interpretation. There is a remote risk of inducing a systemic reaction. To avoid this, most allergists perform epicutaneous (prick) testing first, followed by selected intradermal tests to allergens negative by prick testing. Intradermal skin testing techniques are often used for diagnostic confirmation of hymenoptera (insect venom) or penicillin hypersensitivity, since this method increases sensitivity of the assay for detection of IgE-mediated anaphylaxis. Skin testing with hymenoptera venom or a drug is performed by serial titration, starting with diluted solutions. Special allergenic extracts can be prepared for other allergens (eg, food or latex).
B. In Vitro IgE Antibody Tests
IgE antibody can be detected in serum by immunoassays. Many common atopic allergens are commercially available for in vitro testing. In vitro tests detect allergen-specific IgE antibody in serum. Since IgE-mediated allergy is caused by IgE antibodies bound to mast cells (not by circulating IgE), in vitro tests are generally less sensitive as diagnostic tests than skin testing. Serum immunoassays are not affected by antihistamine therapy but can give false-positive results in patients with very high total serum IgE levels and false-negative results in patients treated with immunotherapy who have significant allergen-specific IgG antibodies. These tests can be significantly more expensive than skin testing, ...