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Allergies

Allergy Tests

Testing for allergies is not straightforward. A positive test does not mean one has allergy and a negative test does not rule out an allergy. A good history establishing the sequence of events is key to selecting the appropriate test.

Skin prick test and Prick-Prick test for allergens

A skin prick test is usually the first test to be done when looking for an allergen. It's quick, and you get the results within about 20 minutes. Skin is pricked with a tiny amount of the suspected allergen to see if there's a reaction. If there is, the skin around the prick will very quickly become itchy and a red, swollen mark called a wheal will appear. Skin tests are done using standardised allergen extracts.

Many allergens do not have standardised extracts, in which case one would do Prick-Prick testing using the suspected food / fruit.

You need to avoid taking anti-histamines and certain other medications before the test. Long acting antihistamines (those that do not cause drowsiness) should be stopped for five days, short acting antihistamines can be stopped 48 hours beforehand.

Two control samples are included to make sure that the test has worked; one of the controls will cause a reaction in all people, and the other should not cause a reaction in anyone. This helps the nurses and doctors ensure the test has been conducted properly.

Intradermal tests

This is used especially in investigation of drug allergy. A tiny amount of diluted drug in question is introduced in-between the layers of the skin to see whether there is a reaction.

Blood test for allergens – Specific IgE ( RAST)

The blood test used to test for allergens is called a specific IgE test, formally known as the RAST test. It is used to measure the number of IgE antibodies in your blood that have been produced by your immune system in response to a suspected allergen. There are around 1000 different specific IgE tests. Many people who don’t have allergic symptoms can have positive tests. The tests need to be carefully selected for them to useful.

Component Resolved Specifc IgE tests

This is also known as component resolved diagnosis (CRD) – this is a very recent development in specific IgE testing, whereby the laboratory can detect IgE to specific pieces of an allergen. This is helpful in teasing out the severity of allergy and helpful in finding out cross reactive allergies.

The ISAC blood test  looks at 112 different components from 52 different allergens in one test, including airborne allergens, food and stinging insect venoms. The test requires a small amount of blood. The results needs to be interpreted in the context of history. It does not have all the allergens.

Patch Testing

A patch test is used if there is suspicion of a delayed allergic reaction. It is most commonly used in the investigation of contact dermatitis, and done by dermatologists with specialist interest. We do not do Patch tests but will refer to our dermatology colleagues if it is needed.

Allergen Challenge

Allergy tests are not perfect. They can be falsely positive when one does not have allergy and sometimes can be falsely negative when one has allergy. When the tests are inconclusive challenge ( food / drug) challenges can be undertaken to clarify presence of absence of allergy. This should only be done in hospital under close medical supervision.

Aspirin challenge (Provocation) test

This test can be done in nasal or oral form in a specific group of patients with asthma who are candidates for aspirin desensitisation treatment.

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