Blood Tests for Allergies

Allergies are caused by an uncommon immune reaction that occurs upon exposure to a normally harmless substance. This response causes your body to produce antibodies, which are proteins that can trigger the release of chemicals such as histamine. The result is itching, respiratory inflammation, or other allergic symptoms. Blood tests for allergies typically measure levels of the antibody IgE, which is responsible for most allergic reactions. For example, the radioallergosorbent test (RAST) exposes a sample of your blood to a suspected allergen and looks for the presence of the IgE antibody. Total serum levels of IgE may also be measured directly.

Purpose of Blood Tests for Allergies

  • To identify or confirm an allergy to a specific allergen—such as dust mites, animal dander, pollens, certain drugs or foods, or stinging insects—particularly if the results of a skin test are inconclusive or skin testing cannot be done
  • To monitor immunotherapy (desensitization)

Who Performs Blood Tests for Allergies

  • A doctor, a nurse or a lab technician

Special Concerns about Allergy Blood Tests

  • None

Before Blood Tests for Allergies

  • No special preparation is needed.

What You Experience

  • A sample of blood is drawn from a vein, usually in your arm, and sent to a laboratory for analysis.
  • The test takes a few minutes to perform.

Risks and Complications of Blood Tests for Allergies

  • None

After Blood Tests for Allergies

  • Immediately after blood is drawn, pressure is applied (with cotton or gauze) to the puncture site.
  • Blood may collect and clot under the skin (hematoma) at the puncture site; this is harmless and will resolve on its own. For a large hematoma that causes swelling and discomfort, apply ice initially; after 24 hours, use warm, moist compresses to help dissolve the clotted blood.

Results of Blood Tests for Allergies

  • Your blood sample is sent to a laboratory for analysis. With RAST, the blood is exposed to small disks, gels, or other materials to which the suspected allergens have been attached.
  • Different techniques may be employed to assess the presence of IgE antibody. For example, RAST uses a radioactive reagent to quantify IgE levels, while a similar technique called EAST (or ELISA) uses color production by an enzyme that tags the IgE.
  • Your doctor will consider the results along with your symptoms, medical history, and the results of other tests.
  • If a specific allergy is diagnosed, your doctor may advise you to avoid the allergen, use an antihistamine or other anti-inflammatory medicine, or have a series of allergy shots to increase your tolerance.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 10 Jan 2012

Last Modified: 28 Aug 2015