Blood Testing - Pinpointing Your Allergic Triggers: Allergies

Content provided by the Faculty of the Harvard Medical School

Blood testing

Blood testing

Although skin tests are the tests of choice for IgE-mediated reactions, a blood test called RAST, which stands for “radioallergosorbent test,” will detect the quantity of IgE antibodies circulating in the blood. The original RAST used an antibody labeled with a radioactive element to detect specific IgE. Modern tests use chemical labels instead of radioactivity to detect specific IgE, but the name “RAST” has stuck.

Blood tests are generally less sensitive than the prick test, and results are not immediately available. But for individuals who can’t interrupt antihistamine or tricyclic antidepressant treatments, or who have a high risk for anaphylaxis with the prick test (because of life-threatening allergies), or whose skin is severely affected by eczema, RAST is an option. The results of RAST take longer to get, and the test costs more than skin tests. RAST is also less sensitive than skin tests, meaning that a positive test is more meaningful than a negative test. However, the blood tests are becoming more sensitive and may soon be as sensitive and specific as skin testing, which remains the gold standard for diagnosing allergies.

What to tell your doctor When you visit your allergist, be prepared to answer the following questions: Do your allergies occur at a particular time of the year? How early in the year do your symptoms start? When do they finish? Are they provoked by molds, dust, or animals? Be as specific as possible. For example, are they provoked by compost or a moldy bathroom or basement? Are they provoked by certain foods? Do other environmental stimuli, such as perfumes or cold air, trigger your symptoms?

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Last updated:	August 21, 2006
Reviewed By:	Faculty of Harvard Medical School

Medical content reviewed by the Faculty of the Harvard Medical School. Harvard Health Publications, Copyright © 2007 by President and Fellows of Harvard College. All rights reserved. Used with permission of StayWell.

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