Allergy blood tests look for substances in the blood called antibodies. Blood tests are not as sensitive as skin tests but are often used for people who are not able to have skin tests.

The most common type of blood test is called radioallergosorbent testing (RAST). It is used to measure the blood level of a type of antibody (called immunoglobulin E, or IgE) that the body may produce in response to particular allergens. IgE levels are often higher in people who have allergies or asthma. RAST may be used for people who cannot have skin tests, such as people who take certain medications (such as some antidepressants) that make the results of skin tests less accurate.

Other laboratory testing methods, such as an immunoassay capture test (ImmunoCAP, UniCAP, or Pharmacia CAP), may be used by your health professional to provide additional information.

A new blood test now available is the mediator release test (MRT). Click here to learn more

A food allergy blood test on a blood sample may be done instead of a skin prick test if a person:

• Has hives or another skin condition, such as eczema, that makes it hard to see the results of skin testing.
• Cannot stop taking a medication, such as an antihistamine or tricyclic antidepressant, that may prevent or reduce a reaction to a substance even when a person is allergic to the substance.
• Has had a severe allergic reaction (anaphylaxis).
• Has had positive skin tests to many foods. Radioallergosorbent testing (RAST) can determine the foods to which a person is most allergic.

How To Prepare
Many medications can affect the results of this food allergy blood test. Be sure to tell your health professional about all the nonprescription and prescription medications you take. You may need to stop taking some medications, such as some tricyclic antidepressants and antihistamines such as cetirizine (Zyrtec), fexofenadine (Allegra), and loratadine (Claritin) before you have an allergy skin test.

Tricyclic antidepressants and antihistamines have no effect on radioallergosorbent test (RAST) results. However, you should not have a radioactive scan (such as a bone or thyroid scan) within a week before a scheduled RAST.

Do not drink alcohol or beverages containing caffeine before you have a RAST. You do not need to restrict foods or other fluids before skin test or a RAST.

(information for the article was obtained from WebMD)