Allergy tests evaluate levels of allergic sensitivity to commonly encountered allergens, which may be foods, pollen, chemicals, or other substances in the environment.
When recurring symptoms in children indicate sensitivity to certain substances in the environment, allergy testing can be used to identify the particular allergens that my be triggering reactions. Using special testing techniques, the child will be exposed to small amounts of specific allergens to determine which of these might provoke a reaction or a "positive" result. Laboratory tests may also be conducted to identify allergens that react with allergy-related substances in the child's blood serum. Positive results from any allergy test may be used to narrow candidates for the allergen responsible for the reaction. Identification of the allergen may help parents avoid exposing their child to the substance and thereby reduce allergic reactions. In addition, allergy testing may be done on children with asthma, eczema, or skin rashes to determine if an allergy is causing the condition or making another condition worse. Allergy tests may also be done before allergen desensitization treatments to ensure the safety of additional exposure.
Allergy is the reaction of the immune system to substances foreign to the body. It is normal for the immune system to respond to foreign microorganisms and particles, like pollen or dust, by producing antibodies against those substances. Antibodies are specific proteins the immune system manufactures to bind to corresponding molecules (antigens) on the cell surfaces of foreign organisms in an attempt to render them harmless. This antigen/antibody reaction is the body's way of protecting itself from invasion by harmful substances and the allergic responses or possible illness that may follow. In some sensitive individuals, excessive antibody production can be triggered by seemingly harmless, everyday substances in the environment. This reactive condition is commonly known as allergy, and the offending substance is called an allergen. Allergic disease arises in the sensitive child from either acute or chronic exposure to certain allergens by inhaling, ingesting, or touching them. Allergic reactions may be dose dependent; that is, longer exposure or exposure to larger amounts of the offending allergen may cause a greater response of the immune system and result in a stronger reaction. Common inhaled allergens include pollen, dust, cat dander, and insect parts from tiny house mites. Common food allergens, all protein-based, include nuts, shellfish, and milk. Allergic reactions can also be triggered by insect bites, molds and fungi, certain prescription drugs, plants such as poison ivy and poison oak, and irritating or toxic substances released into the air.
Allergic reactions involve a special set of cells in the immune system known as mast cells. Mast cells serve as guards in the tissues where the body meets the outside world: the skin; the mucous membranes of the eyes (conjunctiva), nose and throat (nasal and oral mucosa); and the linings of the respiratory and digestive (gastrointestinal) systems. Mast cells produce a special class of antibody, immunoglobulin E (IgE), that coats cell surfaces. Inside the mast cells are reactive chemicals in small
Allergy tests may be performed on the skin or using blood serum in a test tube. During skin tests, potential allergens are placed on the skin and the reaction is observed. In radio-allergosorbent allergy testing (RAST), blood serum is combined with a specific concentration of potential allergens in a test tube, and the mixture is tested for antibody/antigen reactions. Provocation testing involves direct exposure to a likely allergen, either through inhalation or ingestion. It is sometimes performed to determine if symptoms develop on exposure to allergens identified in skin or RAST tests.
The range of allergens used for allergy testing is chosen to reflect possible sources in the environment and may include the following:
- pollen from a variety of trees, common grasses, and weeds
- mold and fungus spores
- house dust
- house mites
- animal skin cells (dander) and saliva
- food extracts
- insect venoms
Skin testing is the most common type of allergy test. There are two forms of skin tests: percutaneous and intradermal. In percutaneous or prick testing, a drop of each allergen to be tested is placed on the skin, usually on the forearm or the back. A typical battery of tests may involve two dozen allergen drops, including a drop of saline solution that should never provoke a reaction (negative control) and a drop of histamine that should always provoke a reaction (positive control). A small needle is inserted through the drop to prick the skin below. A new needle is used for each prick. The sites are examined over the next 20 minutes for evidence of swelling and redness, indicating a positive reaction. In some instances, a tracing of the set of reactions may be made by placing paper over the tested area. Scratch testing, in which the skin is scratched instead of punctured, is used less often, but the principle is the same.
Intradermal testing involves directly injecting allergen solutions into the skin. Separate injections are made for each allergen tested. Observations are made over the next 20 minutes. As in percutaneous tests, a reddened, swollen spot develops at the injection site for each substance to which the child is sensitive. Skin reactivity can be seen for allergens whether they usually affect the skin. In other words, airborne and food allergens that are inhaled or ingested are capable of causing skin reactions when contact is made with mast cells.
Radio-allergosorbent testing (RAST) is a laboratory test performed on those who may be too sensitive to risk exposure to allergens through skin testing or when medications or skin conditions make testing unreliable. RAST testing involves obtaining a blood sample, usually venous blood from a vein in the arm. The sample will be centrifuged in the laboratory to separate the antibody-containing serum from the blood cells. The serum is then exposed to allergens bound to a solid-phase medium. If antibodies against a particular allergen are present, those antibodies will bind to the solid medium and remain attached after being rinsed. The antigen/antibody complex can be detected in the laboratory by adding specific immunoglobulins that are linked with a radioactive dye. The test is read by locating radioactive spots on the solid-phase medium, and a positive result is reported in each test in which reactive allergens are found.
Testing for food allergies is usually done through diet by a process of elimination, that is, by removing the suspect food from the diet for two weeks and then eating a single portion of the suspect food, followed by careful monitoring for symptoms. A slightly different method is to eat a simple, bland, prescribed diet for a period of two weeks, removing all possible food allergens. Suspect foods are then added to the diet one at a time and the individual is observed for reactions.
Provocation testing is done in some cases to confirm associations between exposure to certain allergens and the subsequent development of symptoms when skin testing or RAST tests have indicated possible sensitivity. In provocation challenges, the skin, nasal and oral mucosa, and lining of the lungs and gastrointestinal tract are exposed to suspected allergens. A purified preparation of the allergen is inhaled or ingested in increasing concentrations to determine if it will provoke symptoms. Oral food challenges with foods are more tedious than inhalation testing, since full passage through the
While allergy tests are quite safe for most people, the testing involves additional exposure to allergens. The possibility of causing an exaggerated allergic response, a dangerous condition known as anaphylaxis, does exist. Anaphylaxis can result in difficulty breathing and a sharp drop in blood pressure. Individuals who have had prior anaphylactic episodes should inform the testing clinician. Skin tests should never include a substance to which the individual has had severe allergic reactions or that has previously caused anaphylaxis.
Provocation tests may provoke an allergic reaction by exposing the individual to reactive allergens. Treatment medications such as injectable antihistamines should, therefore, be available during and following the tests, to be administered if needed.
Skin testing is preceded by a brief examination of the skin. The patient should refrain from using anti-allergy drugs for at least 48 hours before testing. Prior to inhalation testing, children with asthma who can tolerate it may be asked to stop asthma medications. Testing for food allergies usually requires the child to avoid all suspect food for at least two weeks before testing.
The RAST test will usually require that a venous blood sample be drawn to obtain sufficient serum for the test. Parents can explain the procedure briefly to the child ahead of time to help reduce fears and encourage cooperation.
Skin testing does not usually require any aftercare. A generalized redness and swelling may occur in the test area, but it will usually resolve within a day or two.
Inhalation tests may cause delayed asthma attacks, even if the antigen administered in the test initially produces no response. Severe initial reactions may justify close professional observation for at least 12 hours after testing.
If a blood sample has been drawn for RAST testing, a bandage may be applied to the venipuncture site to help keep it clean and to stop slight bleeding that may occur. Unusual bleeding or bruising of the site should be reported to the pediatrician.
Children should be observed closely for signs of allergic reactions after allergy testing using skin tests, inhalation tests, or provocation tests.
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. Because patients are monitored following allergy testing, an anaphylactic reaction is usually recognized and treated promptly to reverse the condition. Occasionally, a delayed anaphylactic response can occur that requires immediate care. It is critical that physicians provide education about how to recognize anaphylaxis and tell patients what to do if it occurs at home.
Lack of redness or swelling on a skin test indicates no allergic response. In an inhalation test, the exhalation capacity should remain unchanged. In a food challenge, no symptoms should occur.
Presence of redness or swelling, especially over 5 mm (0.25 inch) in diameter, indicates an allergic response. This reaction does not mean the substance actually causes the child's symptoms, however, since he or she may have no regular exposure to the allergen. In fact, the actual allergen may not have been included in the test array.
Following allergen inhalation, reduction in exhalation capacity of more than 20 percent, and for at least 10 to 20 minutes, indicates a positive reaction to the allergen and the sensitivity of the individual being tested.
Gastrointestinal symptoms within 24 hours following the ingestion of a suspected food allergen indicates a positive response and sensitivity to that food allergen.
Parents who are already confronted with their child's allergies may be reluctant to have the child undergo testing. Physicians and medical personnel can assure parents that careful observation is involved in testing procedures and that allergens to which the child may have had severe reactions will be avoided. Appropriate medications will be available to treat the child immediately if a reaction is provoked by testing. It is important for parents to tell the child how the tests will be done so that the child is not anxious or apprehensive, which in some cases may increase the likelihood of false positive reactions.
Allergen—A foreign substance that provokes an immune reaction or allergic response in some sensitive people but not in most others.
Antibody—A special protein made by the body's immune system as a defense against foreign material (bacteria, viruses, etc.) that enters the body. It is uniquely designed to attack and neutralize the specific antigen that triggered the immune response.
Antigen—A substance (usually a protein) identified as foreign by the body's immune system, triggering the release of antibodies as part of the body's immune response.
Antihistamine—A drug used to treat allergic conditions that blocks the effects of histamine, a substance in the body that causes itching, vascular changes, and mucus secretion when released by cells.
Histamine—A substance released by immune system cells in response to the presence of an allergen. It stimulates widening of blood vessels and increased porousness of blood vessel walls so that fluid and protein leak out from the blood into the surrounding tissue, causing localised inflammation of the tissue.
Immunoglobulin G (IgG)—Immunoglobulin type gamma, the most common type found in the blood and tissue fluids.
Intradermal—An injection into a deep layer of skin.
Mast cells—A type of immune system cell that is found in the lining of the nasal passages and eyelids. It displays a type of antibody called immunoglobulin type E (IgE) on its cell surface and participates in the allergic response by releasing histamine from intracellular granules.
Gordon, Susan, et al. Allergy Methods and Protocols. Totowa, NJ: Humana Press, 2004.
Kemp, Stephen F., and Richard F. Lockey, eds. Diagnostic Testing of Allergic Disease. NY: Marcel Dekker Inc., 2000.
"Allergies." Harvard Medical School Consumer Information. Available online at <http://www.intelihealth.com> (accessed October 10, 2004).
"ImmunoCAP Specific IgE Blood Test." Quest Diagnostics. Available online at <www.questdiagnostics.com/hcp/topics/immunocap/immunocap.html> (accessed October 10, 2004).
L. Lee Culvert