Allergy Tests Health Article

Preparation

Skin testing is preceded by a brief examination of the skin. The patient should refrain from using allergy medications for at least 48 hours before testing.

Immunoassay and RAST tests require a sample of blood, which is obtained via venipuncture by a nurse or phlebotomist followng standard precautions for the prevention of transmission of bloodborne pathogens.

Aftercare

Patients receiving skin tests should be monitored for 30 minutes following testing, and treated promptly should they develop signs of a severe allergic reaction. Occasionally, a delayed anaphylactic response may occur that will require immediate care; therefore, patient education regarding how to recognize delayed anaphylaxis is vital. The generalized redness and swelling that may occur in the skin test area will usually resolve within a day or two. More severe reactions may require topical or antihistamine therapy.

Inhalation tests may cause delayed asthma attacks, even if the antigen administered in the test initially produced no response. Severe initial reactions may justify close professional observation for at least 12 hours after testing.

Complications

Intradermal testing may inadvertently result in the injection of the allergen into the circulation, with an increased risk of adverse reactions. Inhalation tests may provoke an asthma attack. Exposure to new or unsuspected allergens in any test carries the risk of anaphylaxis.

Skin testing

Lack of redness or swelling on a skin test indicates no allergic response. A wheal (an area of redness and swelling) exceeding 7 mm in diameter or larger than the histamine control, has a higher diagnostic value than smaller wheals.

Immunoassay testing

SPECIFIC IGE. Quantitative specific IgE cutoffs are the lowest radioactivity levels encountered in the highest dilution of standard used. The IgE level may be reported in RAST units, or as positive, equivocal, or negative.

TOTAL IGE. The upper limit of normal for total IgE is highly age dependent for children. The upper limit increases over the first 10 years, then levels off. The cutoff for adults varies with the test methodology. For the PRIST test the cutoff is approximately 25 kU/L IgE when the standard used is traceable to the 2nd International Reference Preparation of the World Health Organization.

Inhalation testing

In an inhalation test, the exhalation capacity should remain unchanged. Following allergen inhalation, reduction in exhalation capacity of more than 20%, and for at least 10-20 minutes, indicates a positive reaction to the allergen.

Food testing

In a food challenge, no symptoms should occur. Gastrointestinal symptoms within 24 hours following the ingestion of a suspected food allergen indicates a positive response.

Health care team roles

Allergy tests are ordered by a physician. Skin tests and inhalation tests are performed in a doctor's office by a nurse or healthcare worker specially trained to administer the test. A physician, nurse, or physician assistant should monitor patient for signs of anaphylaxis for 30 minutes after testing.

In vitro allergy tests are performed by clinical laboratory scientists CLS(NCA)/medical technologists MT(ASCP). Results are interpreted by a physician usually a specialist in immunologic and allergic diseases.

KEY TERMS

Allergen—A substance that provokes an allergic response.

Anaphylaxis—Increased sensitivity caused by previous exposure to an allergen that can result in blood vessel dilation (swelling) and smooth muscle contraction. Anaphylaxis can result in shock caused by a sharp fall in blood pressure.

Antibody—A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.

Antigen—A foreign protein to which the body reacts by making antibodies.

Histamine—A chemical released by mast cells that causes vasodilation and bronchial constriction.

Mast cells—A tissue analog of the blood basophil, mast cells are prevalent near small blood vessels in the skin, membranes, and bone marrow. Mast cells bind a type of antibody called immunoglobulin E (IgE) on their surface, and participate in the allergic response by releasing histamine from intracellular granules.

BOOKS

Chernecky, Cynthia C, and Barbara J. Berger. Laboratory Tests and Diagnostic Procedures. 3rd ed. Philadelphia, PA: W. B. Saunders Company, 2001.

Lawlor, G.J., Jr., T.J. Fischer, and D.C. Adelman. Manual of Allergy and Immunology. Boston: Little, Brown and Co.,1995.

PERIODICALS

Parker-Pope, Tara. "Little-Used Simple Blood Test Can Help Screen for Allergies." The Wall Street Journal (September 15, 2000).

"The Use of In Vitro Tests for IgE Antibody in the Specific Diagnosis of Ige-Mediated Disorders and in the Formulation of Allergy Immunotherapy." The Journal of Allergy and Clinical Immunology 90, no. 2 (August 1992): 263-267.

OTHER

Allergy, Asthma & Immunology Online. <http://www.allergy.mcg.edu>.

American Academy of Allergy, Asthma and Immunology <http://www.aaaai.org>.

Victoria E. DeMoranville