The radioallergosorbent test (RAST) and related tests (e.g. RASP, FAST) are blood tests performed to screen for specific triggers of an allergy. The RAST is one of several types of allergy tests given to identify one or more allergens.

By identifying an allergy's trigger, steps can be taken to limit exposure to those triggers (allergens) or to reduce symptoms through immunotherapy. A RAST requires blood to be drawn from a patient and sent to a laboratory for analysis. A physician will look for specific antibodies to determine what may be causing an allergy.

The accuracy of the RAST is comparable to skin tests, which are more commonly given to identify an allergen, but the cost is higher and results take longer to receive. A 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, steroids, ephedrine)
  
  
• Hives or other pervasive skin conditions (e.g., eczema)exist
  
  
• Direct exposure to a suspected allergen may cause anaphylactic shock
  
  
• A patient shows excessive anxiety about a skin test

The test may also be ordered if the patient is an infant or after a positive skin test to many foods.