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Why would I need this test?
A radioallergosorbent test (RAST) is an allergy test that involves collecting blood. A RAST can be performed to test for food allergies and in other circumstances when a skin test - the more common allergy test - cannot be done. Both tests are highly specific, although a skin test is more sensitive, less expensive and produces immediate results.
If you are experiencing symptoms of allergies - such as sneezing, itchy eyes, runny or congested nose, swollen sinuses, coughing, and wheezing - an allergy test may be done to identify which specific substances (allergens) are triggering your allergic reaction.
Common allergens include the waste products from dust mites (tiny bugs you cannot see), proteins found in pet dander, saliva and urine, molds inside and outside your home, tree, grass and weed pollen, and cockroach droppings. Other possible substances that can cause allergic reactions include venom from insect stings, certain foods, food additives, natural rubber latex, or drugs.
How is this test performed?
A technician, doctor, or nurse will use an antiseptic to clean the skin above a vein on your arm. A blood sample will then be drawn. You may feel a brief pricking sensation when the needle is initially inserted. The blood sample will be collected into a tube and then sent to a lab for analysis.
The lab will expose a sample of your blood to different test allergens such as animal dander, grasses, mites, or insects. Allergen-specific IgE antibodies will attach to a substance if you are allergic to it. A radioactive chemical is used in testing to help measure the results.
How long does the procedure take?
A RAST takes about three minutes. Results are usually available within one to two weeks.
What happens if an abnormality is found?
Results are interpreted along with information on your symptom history for a positive diagnosis of allergies. If the results are incomplete, your doctor may order a skin test or provocation test.
If allergen-specific antibodies are found, allergies are suspected. Your healthcare provider may suggest ways to reduce exposure to the allergen and treatment may be recommended to ease symptoms.
Allergy shots or allergy desensitization involves injections of small amounts of the triggering allergen. This works in a way similar to vaccinations or immunizations. That is, your body will react to the injected allergen by building immunity to it. As immunity builds, subsequent environmental exposures to this allergen will not be as likely to trigger an allergic response.
Are there any risks associated with this test?
No risks are associated with this test.
What is a desirable result?
As with skin tests, a negative result on RAST testing is very good way to rule out an allergy to a particular food. However, a positive may not predict that you have an allergy.
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