In Vivo Allergy Testing
In vivo allergy tests fall into two general categories: skin tests and organ challenge (or provocation) tests. Both are designed to confirm hypersensitivity and identify the antigen(s) responsible for the allergic reaction. The most common in vivo allergy tests are outlined below. The efficacy of some in vivo allergy tests has not been firmly established, due to the limited numbers of well-designed clinical trials. Few prospective studies are available, and evidence is primarily in the form of expert opinion.
Skin testing can be utilized to detect immediate hypersensitivity (IgE-dependent reactions) and delayed hypersensitivity (cell-mediated immune reactions). The two major methods of skin testing for IgE-mediated disease include the prick-puncture test and the intradermal test. A positive response to skin testing is typically indicated by the presence of a wheal and/or flare at the test site. Scratch testing is no longer a recommended allergy testing procedure, due to reproducibility issues and the high incidence of false-positive reactions.
Skin testing is contraindicated in patients with severe dermatographism (allergy in which a pale, raised wheal is produced when skin is scratched), ichthyosis (condition in which skin is dry and scaly, resembling fish skin) or generalized eczema; in patients who cannot be withdrawn from medications that interfere with skin testing (such as long-acting antihistamines and tricyclic antidepressants); and in patients who have a history of a previous systemic reaction to skin testing.
Prick/puncture tests are used for confirmation of clinical immediate hypersensitivity induced by inhalant and food allergens. Skin prick/puncture tests are generally considered the most specific screening method for detecting the presence of IgE antibodies in patients with appropriate exposure histories. These tests may also be used in the diagnosis of drug and chemical hypersensitivity reactions. Prick/puncture tests are generally less sensitive than intradermal testing. For inhalant allergies, prick/puncture tests have been shown to correlate better with the presence of clinical allergy. Skin testing is considered the gold standard for the diagnosis of IgE-mediated allergic disease. The Joint Task Force of Allergy, Asthma, and Immunology recommends skin prick/puncture tests as the primary test for the diagnosis of IgE-mediated allergic diseases.
Intradermal or intracutaneous tests are generally used when increased sensitivity is the main goal of testing (i.e., when prick/puncture tests are negative despite a compatible history of exposure). Intradermal tests are more sensitive but less specific than prick/puncture tests for most allergens but are superior to other skin tests for assessing hypersensitivity to hymenoptera (stinging insects) and penicillin or allergens of lower potency.
Repeat skin testing with multiple antigens is not indicated on a regular basis (e.g., yearly). Indications for repeat testing include changing symptoms, new exposures, or 3–5 years of venom immunotherapy.
Patch testing is used to determine the presence or cause of delayed hypersensitivity reactions originating on the skin. It is primarily used to assess allergic contact dermatitis, an eczema-type, immunologically-mediated skin reaction which is largely cell-mediated but may contain an IgE-mediated component. The clinical utility of patch testing to identify allergic reactions other than those originating on the skin (such as inhalants or food allergens) has not been determined. It is estimated that 20–30 antigens used in the panel of patch tests will identify between 50% and 70% of the clinically relevant causes of contact dermatitis.
Certain substances may elicit an allergic reaction only when exposed to light. In photo patch testing, the suspected chemical or medication is applied in two separate areas. One of the areas is exposed to a range of ultraviolet type A light and then examined for the presence of a reaction. Testing is considered positive if only the area that has been exposed to the ultraviolet light demonstrates an allergic reaction.
Oral challenge may be used to confirm or diagnose IgE-mediated hypersensitivity to specific foods, food additives and preservatives, or drugs. Food challenge is time-consuming and associated with the potential for anaphylaxis. Simpler measures, such as skin tests and elimination of suspected foods from the diet, are typically tried first. If skin tests are negative or equivocal and inconsistent with a history suggestive of food allergy, and symptoms abate after elimination of suspected foods, one food at a time is added back into the diet (open food challenge) until symptoms recur. Blinded, controlled food challenge (by ingestion) may be undertaken when skin tests are negative or inconsistent with a history that suggests food allergy. Sublingual food allergy testing, in which the food in question is placed under the tongue and not ingested, is an unproven testing method (see “provocation-neutralization,” below). Double-blind food challenges are typically reserved for a select subset of patients.
Drug provocation/ bronchial challenge testing is typically undertaken only if the need to confirm or exclude hypersensitivity outweighs the risk of severe reaction. This may occur in patients who have a history of allergy to a particular drug for which there is no effective alternative but who need that drug for treatment. Bronchial challenge testing is used in the diagnosis and management of asthma to quantify allergic airway responsiveness to pharmacological agents, such as methacholine or histamine. Bronchial provocation/challenge testing with extracts of common aeroallergens such as dust or ragweed, however, has no established clinical value and offers no additional clinical information beyond that obtained by a well-taken clinical history and a carefully performed skin test.
Serial endpoint titration (SET) is a variation of intradermal skin testing in which increasing doses of antigen are used to determine the concentration at which the reaction changes from negative to positive (i.e., the endpoint). SET has been used as an alternative to skin prick testing or in vitro testing and has also been used to guide initiation of immunotherapy, with the endpoint dilution used as the starting dose. Although not considered a replacement for skin testing, SET may be indicated for determination of a safe starting dose for testing or immunotherapy when there is potential for the specific allergen in question to produce a severe systemic reaction or anaphylaxis (such as with bee venom).