Allergy Testing

What is Allergy?

The term allergy is used to describe a particular response by the body's immune system to a substance in the environment (including foods). This response occurs in predisposed individuals and results in the development of a particular antibody (IgE) against the substance. The next time the person meets this substance the antibody reaction causes the release of certain chemicals into the body. These chemicals cause the symptoms of allergic disease.

Examples of allergic disorders include asthma, eczema, hayfever and year-round nasal symptoms, urticaria (hives or nettlerash) and anaphylaxis.

The tendency to develop allergic disease in this way is inherited. This allergic tendency is called atopy.

What is an Allergen?

Substances which provoke production of IgE and cause allergy in a susceptible person are known as allergens. Almost anything can be an allergen but some substances are much more common allergens than others, for example house dust mites, pollen from trees and grasses, cats, dogs, insects such as wasps and bees, milk, eggs and peanuts. Less common allergens include tree nuts, fruit, and latex. Most allergens are proteins but there are some non-protein allergens such as penicillin and some other drugs. For these to cause an allergic response, they need to be bound to a protein once they are in the body.

What is the difference between Allergy, Intolerance, and Sensitivity?

Although the word Allergy is commonly used to describe any unpleasant reaction to a drug, food, insect sting, or chemical, this can be misleading. The word should only really be used to describe a reaction produced by the immune system when the body meets a normally harmless substance, which has been "remembered" from a previous exposure and subsequently produces the IgE antibody.

Sensitivity is an exaggeration of a normal side effect produced by contact with a substance. For example, the amount of caffeine in a single cup of coffee may cause palpitations and trembling in a sensitive person, where this would normally only occur after far higher doses of caffeine.

Intolerance happens for a variety of reasons including the production of different types of antibody against the substance (not the IgE antibody produced in allergy), or because your body does not produce sufficient quantities of a particular enzyme/chemical, which is needed to break down a food and aid digestion.

How do you know if you have an allergy?

The most useful tool in deciding whether someone is allergic is to take an 'allergy history'. A good allergy clinician can usually identify the likely allergens from the history alone, and allergy tests are often not needed. However there are occasions when tests can be useful to confirm the diagnosis. This is especially important if you have had a severe reaction and if there is any confusion as to whether your symptoms are caused by a true allergy (involving the production of IgE) or whether some other process is involved.

There is no point in having an allergy test if either the sufferer or the doctor is not prepared to act on the results.

What Allergy Tests are available?

The type of test to be carried out will depend upon your symptoms, the condition of your skin, and any medication you are taking.

Skin Prick Testing

This test measures specific IgE attached to cells in the skin. This is probably the most commonly used allergy test and is appropriate for inhaled and ingested (eaten) allergies.

• It is usually carried out on the inner forearm
• If the patient has bad eczema on the area under the test then the test can be performed on the back
• The test allergens are selected in accordance with the patient's history
• As few as 3 or 4 or up to about 25 allergens can be tested
• The name of each thing to be tested is written on the arm (or a number may be used)
• A drop of the allergen (extract) solution is placed by the relevant name or number
• The skin is then pricked through the drop using the tip of a lancet - this can feel a little uncomfortable but should not be painful
• The patient needs to avoid taking antihistamines and certain other medications for several days before the test.

 If the test is positive, the skin becomes itchy within a few minutes and then becomes red and swollen with a "weal" in the centre (very much like the reaction to a nettle sting). The weal has a raised edge which slowly expands to reach its maximum size in about 15 minutes, clearing for most people within an hour. The size of the weal varies with the average being 3-5 mm in diameter. It should be noted the size of the weal does not indicate the severity of symptoms. The test tells us that a level of antibodies is present which may be causing your symptoms.

Also included is a negative and positive control. The negative control is a saline (salt-water) solution, to which a response is not expected. If a patient reacts to a negative control, this indicates that the skin is extremely sensitive and that the results from the test need to be interpreted with care. The positive control solution contains histamine, to which everyone is expected to react. Failure to do so means that something (e.g. medicines taken by the patient) is blocking the response to the allergens, and the test is invalid.

The skin prick test introduces such a tiny amount of allergen into the skin that testing is very safe and almost any age group can be tested. The most common side effect is itching at the site of the test. Where there is a history of a severe allergic reaction, skin testing may not be appropriate.

These tests can be carried out on all age groups including babies, although the response will be considerably smaller than in an adult. Skin prick testing is usually the first test recommended when an allergy is suspected. It is a simple, quick and inexpensive form of testing. It can give useful information in all forms of allergy and provides results within 15-20 minutes. Skin tests should be carried out by specially trained nurses or doctors who can treat any side-effects that might arise.

Blood Tests

Blood tests measure the amount of IgE circulating in the blood. The test is carried out on a small sample of blood, usually taken from a vein in the arm in the usual way. The sample is then sent to a laboratory and the results are available in 7 to 14 days.

These tests are particularly useful when the patient has a risk of severe allergic reaction, when extensive eczema makes skin prick testing impractical, when antihistamine medication cannot be stopped because of severe symptoms, or when unusual and rare allergens are suspected.

IgE results are reported either as 'Classes' ranging from 0 (negative) to 6 (extremely high), or as actual levels. Interpreting the results requires care and experience and should always be done by a trained clinician in conjunction with the clinical history.

Some IgE blood-testing kits have now become available for the patient to use at home. IgE test results are not easy to interpret; very occasionally false positive and false negative results are possible, and the presence of IgE in a test does not necessarily mean that the substance is actually causing the patient's symptoms. For these reasons, home testing of IgE is not recommended without reference to either a healthcare professional or suitable support information.

Patch Testing

This test is performed in cases of contact dermatitis (Eczema) where allergy is suspected.

The allergens are prepared in appropriate concentrations in white soft paraffin (e.g. Vaseline) and are then spread on to discs, 1cm in diameter. The discs (which are usually made of special metal which cannot themselves provoke a reaction) are placed on the skin, usually on the back, and are kept in place by hypoallergenic tape.

The skin is marked appropriately and the patient is asked to keep the skin dry. The patches are left in place for 48 hours. After this time the discs are removed, the skin is examined and any redness or swelling is noted. The skin is re-examined after a further 48 hours for any remaining local redness or swelling.

The interpretation of this form of testing is not as simple as it sounds and tends to be carried out by dermatology departments in hospitals. The symptoms of contact dermatitis need to be brought under control before patch testing can be carried out, or the results will be unreliable. Steroid creams need to be stopped for 3-4 weeks before testing as they may suppress the test response.

Can we test for Food Intolerance?

IgG blood tests have become available which have shown some promising results in identifying the foods that may be responsible for causing symptoms in some cases of migraine, arthritis and irritable bowel syndrome. However food intolerance can occur in various ways and this test will not identify all types of food intolerance. IgG tests are not tests for true food allergy.

The 'gold standard' test for food intolerance remains the elimination and challenge diet (see Food Intolerance fact sheet).

Are there other forms of Testing?

Other (Non-conventional) Allergy Tests are not considered to be relevant, standardised or repeatable and are considered to have no place in the diagnosis of allergy. These include Applied Kinesiology (measures muscle strength), Auricular Cardiac Reflex Method (measures strongest pulse at wrist), Hair Analysis, Leukocytotoxic Tests and Vega Testing ( measures the electromagnetic fields produced by the sufferer).

If in doubt, always request evidence of the results of approved clinical trials.

Updated 5/2009