Allergy Blood Testing
A blood sample drawn from a vein in your arm
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, as well as 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.
A total IgE test measures the overall number of IgE antibodies in the blood, while a specific IgE test measures for a response to individual allergens. Each allergen-specific IgE antibody test performed is separate and very specific; for example, it will test honeybee versus bumblebee, egg white versus egg yolk, and 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 healthcare 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 healthcare 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 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?
- Red itchy eyes
- Coughing, nasal congestion, sneezing
- 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.
What other tests are available for allergy testing?
Skin prick or scratch tests, patch tests, and oral food challenges are usually performed by an allergist or dermatologist. Your health care provider 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.
My allergy test was negative, but I continue to have symptoms. What else could it be?
You could have an allergy-like condition that is not mediated by IgE for which there are no specific laboratory tests. You might 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. Or it could be another disease that is causing allergy-like symptoms. It is important to investigate your individual situation with your healthcare provider's assistance. Test results alone cannot diagnose allergies. Results from any type of allergy test have to be interpreted along with your medical history by a healthcare practitioner who is trained to diagnose allergies specifically.
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.
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 won't work for food and the treatment, which usually consists of years of regular injections, may need to be continued indefinitely.