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Allergy Testing

18 Sep 2008 8:49 PM - Australasian Society of Clinical Immunology and Allergy (ASCIA)

Which allergens should be tested for?

Allergy testing is usually performed in people with suspected hay fever (allergic rhinitis), asthma or reactions to stinging insects or food allergy. In people with hay fever or asthma, allergy testing usually includes house dust mite, cat and dog dander (perhaps other animals if contact occurs), mould spores, pollen from relevant grasses, weeds or trees and in some cases, occupational allergens. Testing can also be used to confirm suspected allergies to foods or stinging insects.

It is important to note that:

  • Allergy test results cannot be used on their own and must be considered together with your medical history
  • Medicare rebates are available for Skin Prick tests or RAST (blood) tests in Australia
  • In some cases, you may be referred to an allergy specialist for further detailed assessment.

Skin Prick testing

Skin prick testing is the most convenient and least expensive method of allergy testing. As results are available within 20 minutes, this allows you to discuss the results with your doctor at the time of testing. Skin prick testing has been shown to improve the accuracy of diagnosis in published clinical studies.

Skin prick testing is most commonly performed on the forearm, although the back is sometimes used. The arm is first cleaned with alcohol. A drop of commercially-produced allergen extract is placed onto a marked area of skin. Using a sterile lancet, a small prick through the drop is made. This allows a small amount of allergen to enter the skin.

If you are allergic to the tested allergen, a small mosquito-like lump will appear at the site of esting over 15-20 minutes. Skin tests are slightly uncomfortable, but are usually well tolerated, even by small children. Local itch and swelling normally subsides within 1-2 hours. More prolonged or severe swelling may be treated with an oral antihistamine, topical corticosteroid cream and an ice pack. Occasional people will experience feel dizzy or light-headed and need to lie down. Severe allergic reactions from allergy testing in asthma or hay fever are very rare.

Skin Prick testing should only be performed by a health professional who has been trained in the procedure, and who knows how to interpret the results. A doctor with knowledge of the management of acute allergic reactions should be present, with access to resuscitation facilities.

Medications with antihistamine-like actions (antihistamines, some cold remedies, some antidepressants) should not be taken for 3-7 days before testing as these will interfere with the results of testing. You may also be advised to avoid creams and moisturisers on your forearms for a similar period of time to reduce the likelihood that allergen extracts will "run" into each other.

Skin Prick testing is not a reliable way of confirming suspected reactions to aspirin or food additives, and you will need to discuss such concerns with your doctor.

Alternative skin testing methods

Alternative skin testing methods such as scratch testing have generally been abandoned because of greater patient discomfort. Whilst it is not widely used in Australia and New Zealand, intradermal skin testing is sometimes used to test for allergies to antibiotic drugs or stinging insects, when greater sensitivity is needed.

RAST (blood) testing

Antibodies (called IgE) directed against specific allergens can be measured with a blood test, commonly referred to as RAST tests. These tests are often performed when skin testing is not easily available, when skin conditions such as severe eczema exist, or when a person is taking medications (such as antihistamines) that interfere with accurate testing.

Total IgE testing

Meaurement of total IgE antibody can be estimated from a blood sample. Total IgE is often (but not always) raised in people with allergies. High IgE antibody levels are also found in internal parasites, eczema and some rare conditions. High IgE levels do not prove that symptoms are due to allergy, and a normal IgE level does not exclude allergy. Therefore, measuring total IgE levels has a limited role to play in allergy testing.

Eosinophil counts

Eosinophils are specialised white cells that are designed to kill worms and parasites. They also can cause inflammation in the tissues in allergy. High levels are sometimes seen in blood samples from people with hay fever, asthma and atopic eczema, as well as in a number of less common conditions. However, a high eosinophil count does not prove that symptoms are due to allergy, and a normal eosinophil count does not exclude allergy. Therefore measuring eosinophil counts has a limited role to play in allergy testing.

Patch testing

Patch testing is useful for testing for contact allergic dermatitis, such as that triggered by nickel metal, cosmetic preservatives or various plants. Using hypoallergenic tape, commercial standardised allergen paste is applied to a rash free area of skin, most commonly the back. The tapes are normally left in place for 48 hours and kept dry for the entire time. The test site is then "read" at different time intervals. An eczema-like rash can indicate sensitivity to a particular allergen.

Challenge testing

Challenge testing may sometimes be required to confirm diagnosis when the cause of a severe allergic reaction has not been confirmed. This will normally only be performed using foods or medications under the supervision of a specialist in allergy and clinical immunology with appropriate resuscitation facilities available.

Unproven methods

There are several methods that claim to test for allergy. These include cytotoxic food testing, kinesiology, Vega testing, electrodermal testing, pulse testing, reflexology and hair analysis. These tests have not been scientifically validated and may lead you to take unnecessary, costly and (in the case of some changes in diet) dangerous avoidance strategies. No Medicare rebate is available in Australia for these tests and the use of these methods is not advised.

Source: © ASCIA 2003 Last updated 27 February 2003

The Australasian Society of Clinical Immunology and Allergy (ASCIA) is the peak professional body of Clinical Allergists and Immunologists in Australia and New Zealand.

Disclaimer:

ASCIA Education Resources (AER) information bulletins have been peer reviewed by ASCIA members and represent the available published literature at the time of review . It is important to note that information contained in this bulletin is not intended to replace professional medical advice. Any questions regarding a medical diagnosis or treatment should be directed to a medical practitioner.

References:

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