Dr Leigh Martens

General Practitioner

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Release Version: Beta

Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Allergy Blood Testing


At a Glance

Why Get Tested?

To help diagnose 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 health care provider 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?

Immunoglobulin E (IgE) is a class of antibody (immune protein) associated with allergic reactions. It is normally found in very small amounts in the blood. This test measures the amount of allergen-specific IgE in the blood in order to detect an allergy to a particular substance.

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, that person becomes sensitized. The person's 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 (a type of white blood cell) in the bloodstream. 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 health practitioner may pick and choose selectively from a long list of individual allergens suspected of causing a person's allergies.

While the traditional method for blood testing was the RAST (radioallergosorbent test), it has been largely replaced with newer IgE-specific immunoassay methods. Some health practitioners continue to refer to all IgE allergy blood tests as RAST even though it is not the exact assay that the testing laboratory uses.

How is the sample collected for testing?

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

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 help diagnose an allergy to a specific substance or substances for a person who 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.

Other types of allergy tests may be performed by exposing a person to different substances under careful medical supervision (see Common Questions #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 one that may be life-threatening such as anaphylaxis. 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 performed 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.

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.

What does the test result mean?

Results of allergy blood testing must be interpreted with care. Even if an IgE test is negative, there is still a small chance that a person does have an allergy. Similarly, 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.

Negative results indicate that a person probably does not have a "true allergy," an IgE-mediated response to the specific allergens tested.

An elevated allergen-specific IgE result indicates that the person tested likely has an allergy. However, 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.

Is there anything else I should know?

Sometimes a health practitioner 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.