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A RAST test (short for radioallergosorbent test) is a blood test used to determine what a person is allergic to. This is different from a skin allergy test, which determines allergy by the reaction of a person's skin to different substances. For assessing the presence of specific IgE antibodies, allergy skin testing, when possible, is the preferred method in comparison with various in vitro tests because it is more sensitive and specific, simpler to use, and less expensive (http://www.mayoclinicproceedings.com/in ... =3978&UID=). [1]

A RAST test, using a person's extracted blood, detects the amount of IgE that reacts specifically with suspected or known allergens. IgE is the antibody associated with the allergic response: if a person exhibits a high level of IgE directed against pollen, the test may indicate the person is allergic to pollen (or pollen-like) proteins. It is worth noting that a person who has outgrown an allergy may still have a positive IgE years after exposure.