Allergy Blood Test

What is an allergy blood test?

An allergy blood test checks for antibodies called immunoglobulin E (IgE) in your blood. Antibodies are proteins your immune system produced in response to harmful substances such as viruses or bacteria. If you have an allergy, your body produces IgE in response to the allergen even if it’s harmless.

Common allergens include:

• Certain foods or ingredients.
• Dust.
• Latex.
• Insect bites and stings.
• Mold.
• Pet dander.
• Pollen.
• Some medications.

They used to call this type of allergy test a radioallergosorbent test (RAST) because it used radioactivity, but now it no longer uses radioactivity.

Are there different types of allergy blood tests?

There are two types of allergy blood tests:

• Total IgE test: Measures the total amount of IgE in your blood.
• Specific IgE test: Measures the IgE in your blood in response to specific allergens.

Allergy blood test vs. allergy skin test: What’s the difference?

Allergy blood tests and allergy skin tests are two of the most common forms of allergy testing. During an allergy skin test, your healthcare provider creates tiny pinpricks in your skin and then applies an allergen to check for a reaction.

Skin tests deliver immediate results, but blood tests take a few days. Skin tests tend to be more accurate, but some people can’t have allergy skin tests. If you have a skin condition such as hives or a rash, or you’re taking antihistamines, allergy skin tests won’t deliver reliable results. In these cases, you may need an allergy blood test.

When is an allergy blood test needed?

You may need an allergy blood test if you have:

• Allergic rhinitis (hay fever), usually the result of pollen, pet allergies or mold (aspergillosis).
• Anaphylaxis, which is a severe and sometimes life-threatening allergic reaction to certain foods, insect stings or other allergens.
• Contact dermatitis, caused by contact with irritants such as chemicals, detergents, poisonous plants or certain metals (such as a nickel allergy).

Allergy symptoms may include:

• Abdominal pain.
• Cough.
• Diarrhea.
• Dry skin.
• Eczema.
• Headache.
• Hives (urticaria) and swelling (angioedema).
• Itchy, red or watery eyes.
• Nausea and vomiting.
• Shortness of breath (dyspnea).
• Skin rash.
• Sneezing.
• Sore throat (pharyngitis).
• Stuffy, itchy or runny nose.
• Swollen lips, tongue, eyes or face.
• Wheezing, tightness in your chest or allergic asthma.

How do I prepare for an allergy blood test?

You don’t need to do anything special to prepare for an allergy blood test. In some cases, your healthcare provider may want you to fast (not eat or drink) before the test. It's important to let your provider know if you take antihistamines. They might ask you to stop taking this medication before your test.

What happens during an allergy blood test?

An allergy blood test only takes a few minutes. A healthcare provider called a lab technician usually takes blood samples in your doctor’s office or a lab.

Here’s what you can expect:

• The phlebotomist (a healthcare provider who draws blood) takes blood from a vein in your arm using a thin needle.
• The needle might cause a mild pinch and some discomfort.
• The phlebotomist fills a collection tube with blood and then removes the needle.
• They place a small bandage on your arm.

Are there any risks with an allergy blood test?

An allergy blood test doesn’t carry serious risks. Some people have bruising, light bleeding or soreness on the inside of their arm. These symptoms usually go away within a day or two.

When will I know the results of my allergy blood test?

You may need to wait several days for the results of your allergy blood test, though it could take more or less time. Ask your healthcare provider when you can expect your results.

What do allergy blood test results mean?

Everyone has some IgE in their blood, but elevated levels may indicate an allergy. Different labs use different brands of allergy blood tests, so the “scoring” system for results can vary from brand to brand. Allergy blood tests don’t indicate the severity of an allergy. So if you do have an allergy, talk to your doctor about the risk of anaphylaxis. You may need to carry an emergency epinephrine injection with you at all times.

Do allergy blood tests ever deliver false-positive results?

About 50% to 60% of all allergy blood tests give false-positive results. False-positive results show you have an allergy even when you don’t. Sometimes false positives happen if your body is reacting slightly to substances in certain foods you’ve eaten recently.

A note from Cleveland Clinic

An allergy blood test checks your blood for certain antibodies that increase if you have an allergy. Your healthcare provider might order one if you have a skin condition, take antihistamines or can’t have a skin allergy test for any reason. It’s important to remember that allergy blood tests often deliver false-positive results and they can’t determine the severity of an allergy. If you have an allergy, talk to your provider about ways to reduce your risk of an allergic reaction.