What is the radioallergosorbent test (RAST)?
A RAST is the common name for the allergen-specific IgE antibody test. IgE antibodies are proteins in the blood that bind to allergens (substances you are allergic to) as part of an allergic reaction. For example, if you are allergic to peanuts, dust mites or pollen, your IgE levels against those substances will be elevated. The RAST test detects those specific IgE antibodies.
Why would I need this test?
If you have allergy symptoms - such as sneezing, itchy eyes, runny or congested nose, swollen sinuses, coughing or wheezing - an allergy test may be done. This will identify which specific substances (allergens) are triggering your allergic reaction. You may also have the RAST test to monitor the effectiveness of immunotherapy, or desensitization, treatment.
Generally, the first-choice allergy test method is skin testing. This is done by placing a small amount of a suspected allergen on or below your skin. If you develop a skin reaction, it means you are allergic. Skin testing is more rapid, reliable and usually less expensive than blood tests. But your doctor may prefer to do the RAST if skin testing cannot be done. This may be because you:
- Have significant dermatitis or eczema, which may obscure reactions during skin testing
Are taking certain medications, which can interfere with skin test results.
- May have a dangerous allergic reaction to the skin test
How is this test performed?
For this test, a blood sample is drawn from a vein in your arm. You may feel a brief pricking sensation when the needle is initially inserted. The blood sample is collected in a tube and sent to a lab for analysis.
The lab will expose a sample of your blood to different test allergens. These may include animal dander, grasses, mites or insects. You and your doctor can decide which groups or panels of allergens you are most likely to react to, based on your allergy history. A radioactive material is used for the test. Most laboratories now use a newer IgE-specific chemical immunoassay. But the name RAST is still often used to refer to IgE allergy tests.
How long does the procedure take?
The blood draw takes just a couple of minutes. Results are usually available within one to two weeks.
What happens if an abnormality is found?
Test results are compared to your symptom history to determine if you have allergies. If the results are incomplete, your doctor may order a skin (or provocation) test. A positive result means that you have allergen-specific IgE antibodies.
If allergen-specific antibodies are found, allergies are suspected. Having the IgE antibodies may only mean that you are sensitized to the allergen. In other words, you were previously exposed, but have not necessarily had an allergic reaction. For this reason, it is important to compare the results of the RAST test with your history of allergies. Your doctor may suggest ways to reduce exposure to the allergen and recommend treatment to ease your symptoms.
What is a desirable result?
As with skin tests, a negative result on RAST testing can rule out an allergy to a particular allergen.
