Allergy Testing

Also Known As

Specific IgE (sIgE)

Allergy test

RAST test

Formal Name

Allergen-specific IgE antibody test

This article was last reviewed on

20 July 2023.

This article waslast modified on 20 July 2023.

At a Glance

Why Get Tested?

To test for suspected allergies.

When To Get Tested?

When you have symptoms of an allergic reaction.

These can include a runny nose, sneezing, wheezing, itchy eyes or skin (hives), tingling around the lips and swelling. If you have a severe reaction like anaphylaxis that your doctor suspects was triggered by an allergic reaction.

Sample Required?

A blood sample taken from a vein in your arm

Test Preparation Needed?

None

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

Immunoglobulin E (IgE) is a protein involved in allergic reactions. It is normally found in very small amounts in the blood and can be attached to the surface of certain specialised cells like mast cells and basophils. Mast cells are found in tissues, for example the gastrointestinal tract and respiratory tract, whereas basophils are found in the blood. Both specialised types of cells contain chemicals or mediators inside like histamine. When these are released they cause the symptoms of an allergic reaction.

IgE is a type of antibody and part of your immune system. It helps to fight against infection. However, sometimes an individual’s immune system will make IgE to harmless substances like pollen, food or animal dander.We don’t yet fully understand why the immune system of some people responds like this, but when it does we refer to these harmless substances as allergens.

When someone with a predisposition to develop allergy is exposed to one of these allergens, their body produces IgE. This IgE can then bind to the surface of their mast cells and basophils in a process called “sensititsation”. When an individual encounters the allergen again, the allergen can bind to IgE stuck to the surface of mast cells and basophils causing them to release the chemicals stored inside the cell. It is the release of these chemicals, such as histamine which causes the symptoms of an allergic reaction. The IgE produced will only bind to one allergen and so is called allergen specific IgE. For example reactions to eating a peanut may be caused by “peanut specific” IgE.

Specific IgE levels in blood can be measured for a particular allergen e.g. peanut, house dust mite, grass pollen. There are many different allergy specific IgE tests available. Some tests can be performed singly and others are available as a panel. For example a “grass mix”. This may be tested if an individual suffers from symptoms when they are outdoors but are unsure what type of grass is planted.

Your doctor should use a detailed clinical history to decide what the possible trigger for allergic symptoms might be. This prevents unnecessary testing and unhelpful results. Allergy specific IgE testing used to be carried out by a radioallergosorbent test and so you may hear this being referred to as RAST testing. However, this method has now been replaced by newer technologies.

Common Questions

How is it used?

The allergen-specific IgE antibody test is carried out to determine if an individual’s symptoms are likely to be caused by an allergy. It can be useful when skin prick tests are not possible, for example if an individual is taking anti-histamines, or has severe dermatitis or eczema.

Specific IgE tests cannot tell you if you have outgrown an existing allergy.

There is currently no clear evidence to indicate the usefulness of allergen-specific IgE to monitor response to immunotherapy.
When is it requested?

The allergen-specific IgE test is usually requested when you have signs or symptoms of allergy.
What does the test result mean?

The meaning of the test result depends on the context in which the test is taken – i.e the clinical context.

Normal or negative results suggest that symptoms are not caused by a reaction to the allergen tested for. However, no one test is perfect and a negative test does not exclude an allergy.

A positive result indicates the individual symptoms may be a result of an allergic reaction. However, not all patients with positive specific IgE results will have symptoms when they encounter the specific allergen.

The amount of specific IgE present does not necessarily predict the severity of a reaction. This is why all allergen specific IgE results must be interpreted within the clinical context of the request and with a detailed clinical history.
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. Elevations in total IgE levels usually mean that an individual will also have high levels of specific IgE. This is called an atopic state.
What other tests are available for allergy testing?

Other tests are usually available in specialist allergy clinics, these include skin prick tests, intradermal tests, and challenge testing. It is important that these tests are carried out in specialist clinics under close medical supervision, as a life threatening anaphylactic reaction, although rare, is possible.

Patch tests are used in the investigation of contact dermatitis. These check for, delayed reactions to substances which are in contact with the skin, eg fragrance in cosmetics, nickel. These reactions generally have different symptoms to allergic reactions and cause an itchy, blistered and dry rash on the skin. They do not depend on IgE and so specific IgE tests are not helpful.

A careful assessment of your symptoms will enable your doctor to choose the appropriate tests for you.
My allergy test was negative, but I am having symptoms. What else could it be?

There are many reasons that an allergy test can be negative, for example you may not have determined the correct trigger for your symptoms. Addtionally, you may have an intolerance that is not caused by IgE for example Coeliac Disease, or it could be another disease that is causing allergy-like symptoms.

It is important to investigate ongoing symptoms with your doctor.
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 stay constant over time.

Although most people cope with their allergy, fatalities do 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 on their own. Avoidance of the allergen and advance preparation for accidental exposure, in the form of drugs such as antihistamines and portable adrenaline injections (when required), is the safest course of action.

Immunotherapy can help decrease symptoms for single allergens when identified. This is currently available for some unavoidable aeroallergens and venoms such as pollens, and bee and wasp stings. New immunotherapy products are also in development for specific food allergy such as peanut.

Immunotherapy is only available at specialist allergy centres.
Why am I told to avoid fresh fruit when my allergy is to tree pollen?

This is due to “oral allergy syndrome”. There are substances in tree pollen that are very similar to those in certain fresh fruits. If your immune system makes IgE to these tree pollen allergens this IgE may also “crossreact” with the fruit proteins. Your body thinks it is detecting and reacting to tree pollen when in fact it is fruit that it is responding to. Oral allergy syndrome in most cases is mild, with tingling or a scratch sensation in the mouth or throat.

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