ELISA & RAST: How Blood Tests Diagnose Allergies

Allergy blood testing can help reveal allergy triggers and aid doctors in providing the best treatment. Allergy blood tests detect and measure the amount of allergen-specific antibodies in the blood. When a person with allergy comes into contact with a trigger, the body makes antibodies against it. These antibodies tell cells in your body to release chemicals, and these chemicals are what cause allergy symptoms. 

Allergy blood tests usually screen for at least ten of the most common allergy triggers and they are particularly helpful in diagnosing food allergies. 


Radioallergosorbent test or RAST is a way of testing a person’s blood to see if they have allergies. This test checks for specific IgE antibodies to determine the substances that a person may be allergic to. It is an alternative to the skin prick test, which on the other hand, determine how a person’s skin would react to specific allergens. 

If the person’s blood contains IgE antibodies that are specific to a certain substance, it means they are allergic to it. These antibodies cause rashes, itching, sneezing, and other symptoms they get when they come into contact with an allergen.

According to the Guidelines for the Diagnosis and Management of Food Allergy in the United States, the original RAST method is now outdated. Instead of using RAST, a physician or an allergist is most likely to order a different blood test called ELISA which stands for enzyme-linked immunosorbent assay. ELISA test also measures the amount of allergen-specific antibodies in your blood.

Allergies can cause an increase in certain types of white blood cells. A doctor may also do a count of a type of white blood cell called eosinophil, especially if he or she suspects that you have allergies. However, keep in mind that there are also other health conditions that can cause an increase in white blood cells.

Why Allergy Tests Are Done

While skin testing is the preferred method, there are some cases wherein the doctor has to order blood testing. Blood testing is recommended if you:

  • Are using a drug that is known to interfere with test results and cannot stop taking it for a few days. These drugs include antihistamines, steroids, and certain antidepressants. 
  • Cannot tolerate needle scratches that are required for skin testing
  • Have an unstable heart condition 
  • Have poorly controlled asthma
  • Have severe eczema, dermatitis, psoriasis, or another severe skin condition
  • Might have an extreme reaction during skin testing or have a history of anaphylaxis

Test Results

A positive result means that allergy-specific antibodies were detected in your blood while a negative result means you probably do not have a true allergy. This means your immune system probably does not respond to the allergen tested. However, it is possible to have a normal (negative) test result and still have allergy.

Side Effects of Allergy Blood Tests

Overall, there are very minor side effects which include swelling and redness at the site where the needle was inserted, pain, and bleeding.