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Allergy Blood Testing

Also known as: RAST Test; Radioallergosorbent Test; Allergy Screen
Formal name: Allergen-specific IgE Antibody Test
Related tests: Total IgE; Complete Blood Count (CBC); White Blood Cell Differential Count; Eosinophil Count; Basophil Count

At a Glance

Why Get Tested?

To screen for allergies; sometimes to monitor the effectiveness of immunotherapy (desensitization) treatment

When to Get Tested?

When you have symptoms such as hives, dermatitis, nasal congestion, red itchy eyes, asthma, or abdominal pain that your doctor suspects may be caused by an allergy

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

This test measures the amount of allergen-specific immunoglobulin E (IgE) in the blood in order to detect an allergy to a particular substance. IgE is a protein associated with allergic reactions; it is normally found in very small amounts in the blood. IgE is an antibody that functions as part of the body's immune system, its defense against "intruders." When someone with a predisposition to allergies is exposed to a potential allergen, such as food, grass, or animal dander, for the first time, they become sensitized. Their body perceives the potential allergen as a foreign substance and produces a specific IgE antibody that binds to specialized mast cells in the skin, respiratory system, and gastrointestinal tract, and to basophils in the blood stream. With the next exposure, these attached IgE antibodies recognize the allergen and cause the mast and basophil cells to release histamine and other chemicals, resulting in an allergic reaction that begins at the exposure site.

Each allergen-specific IgE antibody test performed is separate and very specific: honeybee versus bumblebee, egg white versus egg yolk, giant ragweed versus western ragweed. Groupings of these tests, such as food panels or regional weed, grass, and mold panels, can be done. Alternatively, the doctor may pick and choose selectively from a long list of individual allergens suspected of causing a person's allergies.

The allergen-specific IgE test can be done using a variety of methods. The traditional method that has been used is the RAST (radioallergosorbent test), but it has been largely replaced in most laboratories with the newer IgE-specific immunoassay method. Some doctors refer to all IgE allergy tests as RAST even though this is a specific methodology and may not be the exact assay that the testing lab is using.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

The allergen-specific IgE antibody test is a blood test used to screen for an allergy to a specific substance or substances if a person presents with acute or chronic allergy-like symptoms. This is especially true if symptoms are recurrent and appear to be tied to triggers, such as exposures to particular foods or environments, and if other family members are known to have allergies.

A variety of different types of allergy tests may be performed by exposing a person to different substances under careful medical supervision (see FAQ #1). The usefulness of these tests, however, can be affected by skin conditions, such as significant dermatitis or eczema, and by medications, such as histamines and some anti-depressants. With some tests there is also the potential for severe reactions, including a severe reaction that may be life-threatening. In these cases, the allergen-specific IgE antibody test may be ordered as an alternative, as it is performed on a blood sample and does not have an effect on the person being tested.

The allergen-specific IgE antibody test may also be done to monitor immunotherapy (desensitization) or to see if a child has outgrown an allergy. It can only be used in a general way, however, as the level of IgE present does not correlate to the severity of an allergic reaction, and someone who has outgrown an allergy may have a positive IgE for many years afterward.

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When is it ordered?

One or more allergen-specific IgE antibody tests are usually ordered when a person has signs or symptoms that suggest an allergy to one or more substances. Signs and symptoms may include:

  • Hives
  • Dermatitis
  • Eczema
  • Red itchy eyes
  • Coughing, nasal congestion, sneezing
  • Asthma
  • Itching and tingling in the mouth
  • Abdominal pain, or vomiting and diarrhea

A test may also be ordered occasionally to help evaluate the effectiveness of immunotherapy or to determine whether a child has outgrown an allergy.

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What does the test result mean?

Negative results indicate that a person probably does not have a "true allergy," an IgE-mediated response to that specific allergen, but the results of allergen-specific IgE antibody tests must always be interpreted and used with caution and the advice of the doctor. Even if an IgE test is negative, there is still a small chance that a person does have an allergy.

Elevated results usually indicate an allergy, but even if the specific IgE test is positive, a person may or may not ever have an actual physical allergic reaction when exposed to that substance. The amount of specific IgE present does not necessarily predict the potential severity of a reaction. A person's clinical history and additional medically supervised allergy tests may be necessary to confirm an allergy diagnosis.

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Is there anything else I should know?

Sometimes your doctor will look at other blood tests for an indirect indication of an ongoing allergic process, including a total IgE level or a complete blood count (CBC) and white blood cell differential (specifically eosinophils and basophils). Increases in these test results may suggest an allergy, but they may also be elevated for other reasons.

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Common Questions

1.  What other tests are available for allergy testing?

Skin prick or scratch tests, patch tests, and oral food challenges are usually done by an allergist or dermatologist. Your doctor may also try eliminating foods from your diet and then reintroducing them to find out what you are allergic to. It is important that these tests be done under close medical supervision, as a life-threatening anaphylactic reaction is possible.

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2.  My allergy test was negative, but I am having symptoms. What else could it be?

You could have a genetic hypersensitivity problem, such as sensitivity to gluten with Celiac disease or have an enzyme deficiency, such as a lactase deficiency causing lactose intolerance. It could also be an allergy-like condition that is not mediated by IgE for which there are no specific laboratory tests. Or it could be another disease that is causing allergy-like symptoms. It is important to investigate your individual situation with your doctor's assistance.

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3.  My allergy symptoms are generally mild. How serious is this really?

Allergic reactions are very individual. They can be mild or severe, vary from exposure to exposure, get worse over time (or may not), involve the whole body, and can sometimes be fatal.

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4.  Will my allergies ever go away?

Although children do outgrow some allergies, adults usually do not. Allergies that cause the worst reactions, such as anaphylaxis caused by peanuts, do not usually go away. Avoidance of the allergen and advance preparation for accidental exposure, in the form of medications such as antihistamines and portable epinephrine injections, is the safest course. Immunotherapy can help decrease symptoms for some unavoidable allergies, but they won’t work for food and the treatment, which usually consists of years of regular injections, may need to be continued indefinitely.

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5.  Why am I told to avoid fresh fruit when my allergy is to tree pollen?

There are cross-reactions between some airborne allergens and fruit proteins. Your body thinks it is detecting tree pollen and creates an allergic reaction to the fruit. It is, however, a relatively rare occurrence.

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Ask a Laboratory Scientist

This form enables you to ask specific questions about your tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. If your questions are not related to your lab tests, please submit them via our Contact Us form. Thank you.

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

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