What is a "radioallergosorbant test" or "RAST"?

What is a "radioallergosorbant test" or "RAST"?

Harold Kaiser (Practicing Allergist and Clinical Professor of Medicine, University of Minnesota Medical School) gives expert video advice on: When should I see a doctor if I think I have allergies?; Are allergies genetic?; What are the tests to find out if I have allergies? and more...

The radioallergosorbant test or RAST test as it's more commonly known is a blood test which shows the presence or absence of antigen-specific immunoglobulins. What that really means is it's a blood test which indicates -- as a skin test indicates -- the presence or absence of allergic antibodies in the patient. The blood test is sometimes used in those patients for whom a skin test is not appropriate. These conditions might include: eczema of the skin; fear of being scratched up; a patient who's taking antihistamines which may inhibit the reaction; or some patients are just more comfortable having a blood test drawn than having their skin scratched. The advantage of it is it's a blood draw and the blood is sent off to the lab and one gets a quantitative answer. The disadvantage of it is it takes a longer time. A scratch test or an intradermal test takes 15 to 20 minutes. A RAST test takes overnight or a few days, depending. It's more expensive, typically, and in experienced hands, the scratch test and the intradermal test are more sensitive than the RAST test.