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Allergy blood test (IgE)

Also known as: RAST test; allergy screen; IGE
Formal name: Allergen-specific IgE antibody test

At a Glance

Why Get Tested?

To screen for allergies, to identify particular allergens

When to Get Tested?

When you have symptoms such as a rash, dermatitis, rhinitis (nasal congestion), red itchy eyes or asthma 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?

Immunoglobulin E (IgE) is a protein associated with allergic reactions that is normally found in very small amounts in the blood. IgE functions as part of the body's immune system (its defence against 'intruders').

When someone with a genetic predisposition to allergies is exposed to a potential allergen such as grass pollen or cat fur for the first time, they may become sensitised. Their body sees the allergen as a threat and creates a specific IgE antibody that binds to specialised cells in the tissues (called 'mast cells'), and a type of white blood cell, known as a basophil, in the blood stream. Mast cells are found throughout the body but are highest in the skin, respiratory system and gastrointestinal tract.

If the person is exposed to the allergen again, these attached IgE antibodies recognise the allergen and cause the mast cells and basophils to release histamine and other chemicals, resulting in an allergic reaction that begins at the exposure site.

The allergen-specific IgE antibody test is used to screen for an allergy to a specific allergen. It measures the amount of that suspected IgE antibody in the blood. The specific IgE tests can be for one particular allergen or a group of them and it can be very specific, for example egg white from egg yolk allergies. Groupings of these tests, such as food panels or regional weed, grass and mould panels, can be done. Alternatively, you and your doctor may pick and choose selectively from a long list of individual allergens suspected of causing your allergies.

The allergen-specific IgE test can be done using a variety of methods. The method that has been used and studied for the longest time is the RAST (RadioAllergoSorbent Test), this is not a method currently in use but an allergy test is often wrongly referred to as a RAST test.

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 antigen test is done to screen for an allergy to a specific substance or substances in response to allergy-like symptoms in the patient.

The allergen-specific IgE antibody test may be done when the patient has had a skin prick test which has not helped or has dermatitis or eczema, is taking necessary histamines or anti-depressants, or if a dangerous allergic reaction could be expected from skin prick testing.

The allergen-specific IgE antibody test may also be done to see if a child has outgrown an allergy, although it can only be used in a general way; 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 specific IgE for many years afterward.

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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 or foods.

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What does the test result mean?

Normal negative results indicate that you probably do not have a true allergy to that specific allergen, but the results of the test must always be interpreted and used with caution and with the advice of your doctor. Even if your IgE test is negative, there is still a small chance that you could have an allergy.

Elevated results usually indicate an allergy, but even if your specific IgE test was positive, you may never have an actual physical allergic reaction when exposed to that substance. Also the amount of specific IgE present does not necessarily predict the potential severity of a reaction. Your clinical history and other allergy tests, done under close medical supervision, 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 your total IgE level or your full blood count (FBC) and white blood cell differential (specifically at your eosinophils and basophils - specialised cells of the immune system). Elevations in these tests 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, patch tests and oral food challenges are usually done by an immunologist, 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 such as Coeliac disease (sensitivity to gluten in the diet), or an enzyme deficiency, for example, lactase deficiency which causes lactose intolerance. Alternatively, you may have an allergy-like condition that is not mediated by IgE for which there are no specific laboratory tests. 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 adrenaline injections, is the safest course. Immunotherapy can help decrease symptoms for some unavoidable allergies, but does not work for food allergies and the treatment is very time consuming usually consisting of years of regular injections.

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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 because the antibody it makes against tree pollen will recognise fruit proteins present in apples, pears, peaches and some other fresh fruit. This however, is a relatively rare occurrence.

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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.