An allergy is defined as an acquired hypersensitivity to a substance (allergen) that does not normally cause a reaction. It is essentially a disorder of the immune system that results in an antibody-antigen reaction. The most common manifestations of allergy involve the respiratory tract or the skin. Allergic conditions can cause manifestations such as, but not limited to, eczema, allergic rhinitis, hay fever, bronchial asthma, and urticaria (hives). Severe reaction, such as an anaphylactic response, can result in dyspnea, violent cough, chest constriction, cyanosis, fever, pulse variations, convulsions, and/or collapse.
As the majority of clinically significant environmental, food, and medication allergies are mediated by an IgE antibody immune system response, allergy testing aims to identify an IgE-specific response to the allergen in question through in vivo or in vitro methods. In vivo allergy tests are those that are performed directly on the individual, in contrast to using an individual's serum sample as is characteristic of in vitro tests.
In vivo allergy testing is performed to determine an individual's sensitivity to particular allergens and the degree of the reaction. This testing will provide recommendations for changes in the home/work environment and/or evaluate the necessity of medications and/or immunotherapy to control the reactions. Skin prick testing is the most commonly performed allergy sensitivity test, although the following cutaneous and mucous membrane tests may also be included in an allergy evaluation:
- Skin scratch, puncture or prick testing
- Intradermal testing
- Patch testing
- Photo patch testing
- Photo testing
- Bronchial inhalation challenges
- Food ingestion challenges
- Mucous membrane testing
- Serial endpoint titration (SET) testing (e.g., intradermal dilutional testing [IDT])
In vivo allergy tests must be interpreted in the context of each individual's specific clinical history; the diagnosis of an allergic disorder cannot be based solely on the result of an in vivo sensitivity test. Allergy is a dynamic physiologic response with multiple contributing variables, including, but not limited to, presence of allergen-specific IgE. Although most individuals who experience symptoms of hypersensitivity upon allergen exposure show measurable levels of allergen-specific IgE, some sensitized individuals may be asymptomatic upon exposure to the allergen of interest. Skin tests and other in vivo tests are important and recommended in the identification of allergy, but it should be noted that alone they are not sufficient to diagnose allergy.
SKIN TESTING
Skin tests (scratch, puncture, prick, intradermal) accurately measure an individual's response to certain allergens. A small amount of solution containing different allergens is used. The professional provider either injects or applies the solution to the skin and evaluates the skin reaction. Photo patch testing includes exposing the skin to a dose of ultraviolet light that is sufficient to produce an inflammatory redness of the skin. If the test is positive, a more severe reaction develops at the patch site than on the surrounding skin.
PATCH TESTING
Patch testing is used to differentiate allergic contact dermatitis (ACD) and irritant contact dermatitis (ICD). Patch testing is the gold standard method of identifying the cause of allergic contact dermatitis. A small amount of the suspected allergen is applied to the person's back and evaluated after 48, 72, and 96 hours.
PHOTO PATCH TESTING
Photo patch testing is a form of patch testing used when the reaction to the suspected antigen occurs only in the presence of sunlight. It includes exposing the skin to a dose of ultraviolet light that is sufficient to produce an inflammatory redness of the skin. If the test is positive, a more severe reaction develops at the patch site than on the surrounding skin.
PHOTO TESTS
Photo tests are used to evaluate skin abnormalities (e.g., itching, blisters, and hives) resulting from exposure to sunlight.
BRONCHIAL INHALATION CHALLENGES
Bronchial inhalation challenges are used to determine if an individual has hyper-responsive airways when skin testing sensitivity results are not consistent with an individual's history. Histamine, methacholine, or allergens may be administered in increasing increments until a response is produced.
FOOD INGESTION CHALLENGES
Food ingestion challenges help diagnose allergies to food. The individual ingests the food to which sensitivity is suspected and is observed and evaluated for allergy symptoms. The individual must undergo a risk evaluation determination to find the food challenge category that describes their risk level (i.e., low risk or high risk). Risk evaluation decisions are based on the same types of data that are evaluated in the early food allergy diagnosis process (e.g., penicillin skin test [PST] results, patient history).
Once a risk category is determined, the decision regarding what the most appropriate setting (office, hospital, or short procedure unit) for the oral food challenge is can be made.
For the individual in the low risk category, the settings where injections for allergen immunotherapy are administered should suffice for oral food challenges.
If the challenge is considered high risk (e.g., the individual is asthmatic, the individual has had a previous severe reaction), the challenge should be performed in a more controlled setting where additional interventions to support and reverse anaphylactic shock are available, such as a hospital or short procedure unit.
MUCOUS MEMBRANE TESTING
Conjunctival or nasal challenge testing involves the direct administration of the allergen to the mucosa, to test for suspected localized eye allergy or nasal allergy, respectively. The individual is observed and evaluated for allergy symptoms.
SET TESTING
SET testing is used in conjunction with immunotherapy to determine a starting point for an individual's sensitivity from an immune globulin E (IgE) standpoint for the allergen (antigen) in question. This procedure involves injecting a series of a very small amount of antigen into the individual's arm. Each injection bump, which is called a wheal, is then measured and recorded in millimeters.
The size of the wheal helps the professional provider determine an individual's sensitivity to the allergen and what dosage is needed to safely begin the desensitization process. Generally, it takes three tests (dilutions) per allergen to determine the serial endpoint.
OTHER TESTING
Provocation-neutralization testing determines the presence of allergy to foods, inhalants, and environmental chemicals by exposing the individual to test doses of substances that are administered intradermally, subcutaneously, or sublingually with the goal of either producing or preventing subjective symptoms. The Rebuck skin window test involves scraping the individual's skin, applying a suspected allergen on the scraped area, and measuring the reaction to the allergen. There is insufficient evidence to support the effectiveness of sublingual testing (antigens prepared for sublingual administration), provocation-neutralization testing, and the Rebuck skin window test for the diagnosis and treatment of allergies.
