An allergy can only be treated successfully when the cause of the allergy is known. The radioallergosorbent test (RAST) and related tests (e.g. RASP, FAST) are blood tests performed to determine the specific cause of an allergy.

The RAST looks for specific antibodies called immunoglobulin E (IgE), which are produced in response to an allergen. The body’s immune system produces a number of different antibodies to defend against intruders (called antigens).

An allergy occurs when the body overreacts to a substance that is normally harmless. A person who is allergic to pollen, for instance, produces IgE antibodies to “defend” against pollen molecules that enter the body.

These IgE antibodies bind to white blood cells called mast cells and basophils. When they come into contact with a pollen molecule, mast cells and basophils explode, releasing chemicals such as histamine. Histamine binds with receptors in the nose and other areas of the body, causing allergy symptoms (e.g., sneezing, runny nose, itching).

The best way to treat an allergy is through avoidance, which is possible only when the allergen is identified. The RAST uses radioactive or enzyme markers to detect levels of IgE antibodies in the blood. Every specific allergen is selected and tested separately each time. The test measures levels of particular allergens (e.g., egg white versus egg yolk, giant ragweed versus western ragweed).

These tests measure the amount of the suspected IgE antibody in the blood that reacts to each allergen and reports it as a numerical value. The added radioactive substance essentially binds itself to the allergen-antibody in the patient’s blood and highlights the amount of antibody that is responding to the introduced allergen.

When the RAST test is completed and all the suspected allergens have been tested, the allergen or substance that is causing an allergic reaction should be determined. However, there are circumstances when a retest with other allergens or other tests need to be performed to discover the offending allergen.

The RAST test and other blood tests are generally comparable in accuracy to skin prick tests, with both types of tests producing results that are about 90 percent accurate. However, the RAST may not be as accurate when testing for food allergens because the level of antibody in the blood does not necessarily indicate the severity of the allergic reaction in the specific individual. Even low level test results can be associated with a severe reaction. Instead, the RAST test is used to predict the likelihood of a reaction. The lower the level, the less likely a reaction is to occur.

Although both types of tests are considered precise, skin tests are more commonly used because they are less expensive and can produce results within 20 minutes. Blood test results take longer because blood must be sent out to a laboratory for analysis.

RAST is used instead of a skin test when:

• Certain medications can suppress or interfere with skin test results (e.g., some tricyclic antidepressants, antihistamines [including nasal antihistamines and over-the-counter antihistamines], steroids, ephedrine)
  
  
• Hives or other pervasive skin conditions (e.g., eczema) exist and may obscure results

• Direct exposure to a suspected allergen may cause anaphylactic shock
  
  
• Patient anxiety about skin tests persists

A RAST test may also be ordered if the patient is an infant or after a skin test suggests an allergy to many foods.

Avoiding the allergy causing substance and determining if allergy symptoms cease or persist will ultimately determine if an allergy test was accurate.