Five Facts About RAST for Allergy Diagnosis

June 2022  |  Linda Armstrong  |  ✓  Medically reviewed by: Fabio Iachetti, MD

Fabio Iachetti is a licensed physician with more than 15 years of diverse experience in several disease areas such as allergy, CV, pain, GI, rheumatology, urology, and diabetology. He is a Senior Medical Manager for Allergy in ImmunoDiagnostics Global Medical Affairs at Thermo Fisher Scientific.

Despite the fact that the radioallergosorbent test, aka RAST, is about as cutting edge as a rotary phone, RAST used to be the creme de la crème of allergy diagnostics. In fact, this blood test, which measures specific IgE (more on that later), became so common that many healthcare providers started using this now-antiquated brand name as a generic term. You might say that RAST was to allergy blood testing what Bubble Wrap is to air-cushioned packing materials.

The problem is, however, while specific IgE blood tests have evolved considerably, some people still insist on using old-school terminology. The truth is that RAST is not an umbrella term for modern specific IgE blood tests for allergies. Calling all specific IgE tests RASTs would be like calling all cars Model T Fords. (And doing so makes as much sense as putting gas in a Tesla.)

To set the record straight and ensure we’re all using the correct terminology, here are five facts about RAST testing along with a short primer to help you better understand specific IgE testing.

What is a specific IgE blood test for allergies?

Now that you understand the difference between RAST and modern blood tests for allergies, let’s back it up one step and take a quick look at specific IgE testing in general.

IgE stands for Immunoglobulin E, which is a type of antibody (i.e., a protein produced by the immune system in response to the presence of a foreign substance, aka antigen).^4-5 Your body may produce specific IgE antibodies to an allergen (e.g., pollen, mold, a specific food, etc.) the first time you come into contact with it. This is called a sensitization.⁴

For example, let’s say you inhaled ragweed pollen. Your immune system might have an internal dialog like this: “Yikes! There’s an invader. Release the kraken!” The kraken represents specific IgE antibodies, and once they’re launched, you’re sensitized to this particular pollen.

The next time your body contacts ragweed pollen, your immune system will likely release massive amounts of IgE (a whole school of kraken, if you will). This ultimately leads to the discharge of histamine as a defense mechanism.⁴ This histamine, then, creates allergy symptoms, such as sneezing, watery eyes, anaphylaxis, etc.

Allergies are the most common cause of elevated IgE, so allergy-related specific IgE tests look for IgE in the blood.¹ Along with a detailed medical history and a physical exam, healthcare providers can use specific IgE blood tests to help diagnose allergies.

Testing may also lead to symptom relief. That is, if you know what you’re allergic to, you may be able to reduce your exposure to individual allergens, and this in turn can help relieve symptoms and improve control of asthma, rhinitis, and eczema.³

If you’re ready to make the leap from learning about blood testing for allergies to actually scheduling a specific IgE test for yourself, complete our symptom tracker, which will generate a comprehensive symptom profile you can then review with your healthcare provider to decide if a specific IgE blood test is right for you. Also check out "How to Ask Your Healthcare Provider for an Allergy Blood Test" for talking points for this important conversation.