The Basics of Allergy Testing
The most common allergens are pollen, dust, food, insect stings, animal dander, mold, medications and latex. Allergy testing is important in the diagnosis and treatment of allergies. Determining specific allergens that are a problem for a patient can improve their quality of life by knowing what to avoid on a daily basis. Results from allergy testing can also help develop treatment protocols for patients with allergies that can be treated with immunotherapy.
Food allergies are mediated by IgE antibodies. The initial exposure to a food allergen results in the body forming IgE antibodies. The first exposure can be via ingestion, physical contact, or inhalation of small particles of a food. The IgE antibodies attach to basophils and mast cells in different tissues throughout the body. Upon subsequent exposure to the food allergen, the allergen attaches to the IgE-specific antibodies, signaling the release of histamine and other chemicals from the mast cells. Symptoms of food allergy include swelling or itching of the mouth, hives, vomiting, diarrhea, abdominal cramping, hypotension, throat tightening or trouble breathing and eczema. All of these symptoms could be experienced or just some of them, depending on the distribution of mast cells through the body (NIAID 2010). Allergies to substances other than food are mediated in the same way, but the route of exposure may be via inhalation or contact with mucous membranes instead of ingestion.
Allergy testing is performed in several ways and often a combination of methods is used. Prick skin testing and intradermal testing can be performed by an allergist in their office. Serology testing, or RAST testing, involves taking a blood sample and measuring antibody levels in the serum. Blood samples can be drawn in a doctor’s office or in a lab, depending on a particular allergist’s protocol. A food challenge can also be done under the supervision of an allergist to assess for food allergies.
Skin prick testing involves the placement of a very small amount of an allergen or allergens on the skin and then a needle pricks the skin to allow contact of the allergen with the tissue just below the surface of the skin. A result is considered positive if there is swelling and redness at the test site. This positive reaction indicates that there are IgE antibodies bound to mast cells in the skin. The results are read after a few minutes and the procedure is considered to be fairly safe. A diagnosis of an allergy is not made on a positive skin prick test result alone. A physician usually combines allergy testing results with details from the patient’s history to make a diagnosis. This is relevant in situations when a patient has a positive skin prick test for a food, but no reported suspicious symptoms related to ingestion of that food in the past. Certain medications, including antihistamines, have to be discontinued prior to skin prick testing, as they can interfere with results.
Intradermal skin testing involves injecting a small amount of the allergen under the surface of the skin. The injection sites are monitored for reactions to the potential allergens injected. Intradermal skin testing can be used if a skin prick test is negative, yet there is still a high clinical suspicion of an allergy based on patient history. Intradermal skin testing is thought to have a greater sensitivity than skin prick testing. They can also be used to help confirm a negative diagnosis. As with the skin prick testing, antihistamines and certain other medications must be discontinued for a specific timeframe prior to testing. Skin prick testing is less expensive than intradermal testing.
Radioallergosorbent (RAST) testing is serological testing for the levels of IgE antibody to a particular allergen. A positive test for IgE levels to a certain allergen does not make a definitive diagnosis. The test results are considered along with a detailed patient history to make a diagnosis. (AAAAI 2006)
A food challenge involves feeding a patient a potential allergen food in a very controlled process. Only a small amount of the food is ingested initially and then the amount is slowly increased as long as there are no reactions. This type of allergy testing should only be done under direct supervision of a physician, because if the patient has an anaphylactic reaction and immediate medical care cannot be instituted, it could be fatal. (NIAID 2010)
A person suffering from symptoms suspected to be related to allergies should consult an allergist for proper diagnosis and treatment. Allergy testing is very important to determine what someone should avoid on a daily basis to minimize the potential risk of a life-threatening allergic reaction. A patient’s general practitioner can be contacted for a recommendation or referral if needed. The AAAAI can also be contacted to locate an allergist.