An allergy blood test checks your blood for increased levels of immunoglobulin E (IgE) antibodies. The test can help detect allergies to foods, pets, pollen or other irritating substances. Allergy blood tests can deliver false positive results more than half of the time.
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:
They used to call this type of allergy test a radioallergosorbent test (RAST) because it used radioactivity, but now it no longer uses radioactivity.
There are two types of allergy blood tests:
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.
You may need an allergy blood test if you have:
Allergy symptoms may include:
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.
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:
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.
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.
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.
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.
Last reviewed by a Cleveland Clinic medical professional on 02/07/2022.
Learn more about our editorial process.