A blood sample taken from a vein in your arm
Immunoglobulin E (IgE) is a protein involved 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 and helps to defend against "intruders". When someone with a predisposition to allergies is exposed to a potential allergen (such as food, grass or animal dander), they may become sensitized. Their body sees the potential allergen as a foreign substance and produces a specific IgE antibody that binds to mast cells which are specialized cells in your tissues and a type of white blood cell called basophils which are present in the blood. The mast cells are found in tissues throughout your body but have the highest in concentration in the skin, respiratory system, and gastrointestinal tract. At the time of the next exposure, these attached IgE antibodies recognize the allergen and cause the mast and basophil cells to release histamine and other chemicals which results in an allergic reaction that begins at the exposure site.
The allergen-specific IgE antibody test is used to check for an allergy to a specific allergen. It measures the amount of IgE antibody in the blood for the suspected allergen(s). Each selection is one separate test, and the tests are very specific: honeybee versus bumblebee, egg white versus egg yolk, giant ragweed versus western ragweed. Groups of tests, such as food panels or regional weed, grass, and mould panels, can be measured or you and your doctor can choose from a list of individual allergens which are suspected of causing your allergy.
The allergen-specific IgE test can be done using a variety of methods. The traditional method 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 method 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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
The allergen-specific IgE antibody test is done to test for an allergy (a type I hypersensitivity) to a specific substance or substances when a patient presents with acute or chronic allergy-like symptoms.
The allergen-specific IgE antibody test may be done when skin prick tests are not possible. If the patient has significant dermatitis or eczema or is taking anti-histamines, skin prick tests are difficult to perform and/or interpret.
The allergen-specific IgE antibody test has limited use in checking whether a patient has outgrown an allergy as the test may remain positive even in someone who has long outgrown their allergy. The usefulness of allergen-specific IgE to monitor response to immunotherapy is unclear, and may remain positive even after successful immunotherapy.
When is it requested?
The allergen-specific IgE antibody test is usually requested when you have signs or symptoms that suggest that you have an allergy to one or more substances.
What does the test result mean?
Normal negative results indicate that you probably do 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 on the advice of your doctor. Even if your IgE test is negative, there is still a small chance that you do have an allergy.
Elevated results often indicate an allergy. However, not all patients with a positive specific IgE test will have an actual physical allergic reaction when exposed to that substance – the patient in this case is said to be “sensitised” rather than allergic to the substance. The amount of specific IgE present does not necessarily predict the severity of a reaction, although the higher the level the more likely the patient is to have a reaction of some sort to the allergen. Your clinical history and other allergy tests, done under close medical supervision, may be necessary to confirm an allergy diagnosis.
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 your total IgE level or your full blood count (FBC) and white blood cell differential (specifically at your eosinophils and basophils). Elevations in these tests 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, and oral food or drug challenges are usually done by an allergist or immunologist. Your doctor may also try eliminating foods from your diet and may reintroduce 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.
Patch tests are used in the investigation of 'allergic contact dermatitis', and check for delayed reactions to substances which are in contact with the skin, eg fragrance in cosmetics, nickel. These tests are performed by dermatologists.
My allergy test was negative, but I am having symptoms. What else could it be?
You could have an allergy-like condition or intolerance that is not caused 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. In the case of bowel symptoms after food, there can be other causes such as coeliac disease, an autoimmune reaction to gluten, or lactose intolerance, caused by an enzyme deficiency.
My allergy symptoms are generally mild. How serious is this really?
The severity of an allergic reaction varies greatly from one person to another. They can be mild or severe, vary from exposure to exposure, get better or worse or not change over time. They may be local or involve the whole body. Although most people cope with their allergy, fatalities can rarely occur, mainly in patients with severe food allergy.
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 drugs such as antihistamines and portable adrenaline injections, is the safest course. Immunotherapy can help decrease symptoms for some unavoidable aeroallergens such as pollens, and this treatment usually consists of years of regular injections. This approach does not work for food allergens.
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. This condition is known as the oral allergy syndrome. It is becoming more common, though in most cases is mild, with tingling or a scratch sensation in the mouth or throat.