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To maximize the positive predictive value of allergy testing, a positive test result must correlate with patient history of immediate hypersensitivity, and the immune pathogenesis must be consistent with IgE-mediated disease. IgE antibodies are detected by allergen skin testing or serum IgE testing. These allergy tests are useful for patients with symptoms consistent with allergic disease (eg, rhinitis, asthma, or both), patients who may be eligible for allergen immunotherapy, and patients who have strong predisposing factors for atopy (eg, family history of atopy or ongoing exposure to potential sources of allergen). Patients with diseases that are not IgE-mediated or who do not demonstrate an immediate hypersensitivity reaction (eg, non-anaphylaxis food sensitivities or intolerances, allergies to some drugs, and allergic contact dermatitis) usually do not benefit from this type of testing.
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A. Allergy Skin Tests
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Cutaneous or skin prick allergen testing produces a localized pruritic wheal (induration) and flare (erythema) that is maximal at 15–20 minutes. Such testing is used most commonly to diagnose allergic respiratory disease (rhinitis and asthma) and IgE-mediated allergy to food, drugs (penicillins), and hymenoptera venom. Allergen extracts are available for pollens, fungi, animal danders, and dust mites and are appropriately selected for the patient’s geographic area.
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Allergy skin testing is preferred to serum IgE testing as it detects the presence of IgE bound to mast cells and thus shows biologic activity. Skin testing is generally faster, more sensitive and specific, and less expensive than serum IgE tests. Any drug with antihistaminic effects (eg, H1-antagonists, tricyclic antidepressants, phenothiazines) must be stopped prior (4–5 days) to skin testing to avoid a false-negative result. To avoid a remote risk of inducing anaphylaxis, most allergists perform skin prick testing first, followed by selected intradermal tests to allergens negative by prick testing where the history is compelling. Intradermal skin testing techniques are most often used for diagnosis of hymenoptera venom (stinging insect) or penicillin hypersensitivity, as intradermal testing increases sensitivity for detection of IgE-mediated anaphylaxis. They are not performed to food antigens. Skin testing with hymenoptera venom or a drug is performed by serial titration, starting with the most dilute concentrations. Natural rubber latex skin testing is not available in the United States, but it is standardized in Canada and some European countries.
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B. Serum IgE Antibody Tests
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Allergen-specific IgE antibody can be detected in serum by immunoassays. 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 previously treated with immunotherapy. These tests can be more expensive than skin testing, and results are not immediately available. They can be useful as a diagnostic tool when patients are on antihistamines and unable to discontinue them, when patients have dermatographism, and in food allergen testing where quantitative antibody levels predict a high positive and negative predictive value of reacting to ...