Allergy Blood Test

Urgent safety information

If you have trouble breathing, swelling of the throat, rapid pulse, or feel faint, call 911 or go to the nearest emergency department immediately. These may be signs of a life-threatening allergic reaction (anaphylaxis).

Why this test may be ordered

• Symptoms of allergy: stuffy or runny nose, sneezing, itchy/watery eyes, hives, diarrhea, vomiting, shortness of breath, coughing, wheezing.
• If you cannot have allergy skin testing due to skin conditions, certain medicines, or risk of serious allergic reaction.
• Sometimes for young children because skin testing may be too uncomfortable.

Preparation and collection

You don't need any special preparations for an allergy blood test. You may need to stop taking certain medicines before this test, such as antihistamines, so tell your provider about everything you take. But don't stop taking any medicines unless your provider tells you to.

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

There is minimal physical risk to having an allergy blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

Understanding results

Results from an allergy blood test must be interpreted by your healthcare provider, who will consider your symptoms and medical history. The test measures IgE antibodies; levels can suggest whether you might have an allergy, but they are not diagnostic by themselves.

• Total IgE test result high (screen-positive): You may have some kind of allergy, but this test does not show what you are allergic to or how serious your allergy may be. Limitation: Total IgE test does not identify specific allergens; false positives can occur. Next step: Your provider will use your symptoms and history to decide if further specific testing or referral to a specialist is needed.
• Total IgE test result not high (screen-negative): You are less likely to have an allergy, but allergy is not ruled out. Limitation: Normal total IgE does not exclude allergy; false negatives are uncommon but possible. Next step: Your provider may still recommend specific IgE testing if allergy symptoms are present.
• Specific IgE test result high for a particular allergen (screen-positive): You may be allergic to that allergen, but the amount does not predict how serious your allergy may be. Limitation: Specific IgE test can have false positives; the level does not predict severity. Next step: Your provider may refer you to an allergy specialist or recommend a treatment plan based on the specific allergen and your symptoms.
• Specific IgE test result not high for a particular allergen (screen-negative): You are less likely to be allergic to that allergen. Limitation: False negatives are uncommon but possible; a negative result does not completely rule out allergy. Next step: Your provider will consider your symptoms and may order a skin test or other evaluation.

• Physical risks: minimal; slight pain or bruising at needle site; most symptoms go away quickly.
• False-positive: sometimes results may say you have an allergy when you do not; this may happen if your body has a slight reaction to substances in certain foods that you may have eaten before the test.
• False-negative: it is uncommon for a blood test to show you do not have an allergy when you actually do.
• Total IgE test does not show what you are allergic to or how serious your allergy may be.
• Specific IgE test amount does not predict how serious your allergy may be.

What to do next

If the results from either type of test show that you could have an allergy, your provider may refer you to an allergy specialist or recommend a treatment plan. Your treatment plan will depend on what you are allergic to and how serious your symptoms are. If you're at risk for anaphylactic shock, you'll need to be very careful to avoid the things you are allergic to. You may need to carry an emergency epinephrine treatment (an epinephrine auto-injector) with you at all times.

• Ask your provider if you are at risk for anaphylactic shock and discuss any questions you have about your test results or your allergy treatment plan.
• What do my test results mean in the context of my symptoms?
• Do I need any additional testing, such as a skin test?
• Should I stop taking any medications before the test?
• How accurate are allergy blood tests?

Evidence source

U.S. National Library of Medicine: Allergy Blood Test