Radioallergosorbent test: An allergy test done on a sample of blood. The test is used to check for allergic sensitivity to specific substances. In the test, the sample of blood is mixed with substances known to trigger allergies. The test measures the level of allergy antibodies (specific IgE antibodies) in the blood which are present if there is a allergic reaction. Because Radioallergosorbent test is a mouthful, it is best known as RAST.