What is a RAST test? Is it right for me?

The radioallergosorbent (RAST) test, a blood test done in a laboratory, can be used instead of skin tests to detect allergies. The test is no more accurate than skin tests, but much more expensive. Still, the test is valuable in a few situations.

Some skin conditions (such as severe eczema or atopic dermatitis) might make skin testing impossible, or the physician might suspect that a patient is so sensitive to an allergen that a skin test could be dangerous. An advantage of the RAST test is that it’s not affected by antihistamines or other medications  that the patient may be taking. Variations of the RAST include FAST (fluoro-allergosorbent test) and MAST (multiple antigen simultaneous testing).

Positive skin or RAST tests help narrow the list of suspected allergens, but don’t often point directly to the cause of symptoms. The patient’s history is vital in determining which of the positive skin test or RAST results really contribute to allergy symptoms. A challenge test may also be needed. If the patient’s allergic reactions haven’t been severe, the physician might deliberately expose the patient to one or more substances that produced a positive skin or RAST test. This is particularly true with food allergies .

Learn more about food challenges.

This information has been approved by Rohit Katial, MD  (April 2006).