What Is RAST?

As part of diagnosis for eczema, a RAST test is often recommended. What is RAST? 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.

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. Because there are other tests that help with confirmation, results are best interpreted by a doctor.

The RAST was introduced in the 1970′s and replaced by a superior test in 1989. Advantages of the new test range from: improved sensitivity without loss of specificity, to excellent reproducibility across the full measuring range of the calibration curve. In general, this method of blood testing (in-vitro, out of body) vs skin-prick testing (in-vivo, in body) has a major advantage: it is not always necessary to remove the patient from a anthihistamine medication regimen, and if the skin conditions (such as eczema) are so widespread that allergy skin testing can not be done, RAST tests don’t affect results, it’s more invasive (one venipuncture needle versus many skin pricks with a plastic device).