HealthInfo Waitaha Canterbury

Allergy testing

Whakamātaunga mate pāwera

Allergy is a type of reaction (also called a hypersensitivity reaction). Allergies usually happen within an hour of exposure to an environmental (such as grass pollen), food or medication trigger.

Allergies are controlled by proteins in your immune system called IgE antibodies. The allergy testing described below looks for the types of reaction caused by IgE antibodies.

There are many other types of hypersensitivity. These include chemical sensitivity (such as reactions to household cleaners) and food intolerances (such as dairy or gluten sensitivity).

These types of hypersensitivity reactions are not controlled by IgE. They are called non-IgE mediated hypersensitivity.

Your general practice team will look at your history to tell if the reaction is due to an IgE mediated allergy or a non-IgE hypersensitivity. It is helpful to keep a diary of your symptoms and what triggers them.

Allergy testing is only helpful to confirm a strong suspicion of a specific allergy, such as a peanut allergy.

Skin prick testing

Skin prick testing looks at how your skin responds to certain allergens. A solution containing the allergen (such as cat fur) is placed on your skin and your skin is then pricked. If your immune system has met this allergen before and is sensitised to it, you will develop a red bump or weal within 10 to 20 minutes.

If you have no reaction, it means you are unlikely to be allergic to that substance.

You can have a skin prick reaction to a substance that does not cause you an allergic reaction. This happens when you have a sensitivity to the substance but do not have an allergy to it.

The test will not work properly if you are taking antihistamines. You will need to stop all antihistamines at least 72 hours before your test. The test is also not reliable if performed within four weeks of anaphylaxis or an acute allergic reaction to food.

Skin prick testing is useful for

• some cases of acute food reaction causing anaphylaxis, hives (urticaria) or angioedema (facial swelling) within 30 to 60 minutes of being exposed
• hay fever (allergic rhinitis and conjunctivitis)
• asthma that you only get in certain environments
• penicillin allergy.

Skin prick testing is not useful for

• non-specific rashes
• chronic hives (urticaria)
• eczema (unless there is urticaria associated with it)
• allergic contact dermatitis
• food intolerances (including bloating, diarrhoea and fatigue)
• headaches
• behavioural disorders.

EAST/RAST blood test

This is a blood test that looks for IgE antibodies to specific allergens. If it finds them, it means your immune system is sensitised to those allergens. It performs a similar role to skin prick testing using the same principles described above. So, in most instances you only need to do one or the other test, not both.

EAST/RAST blood tests are useful when you cannot do a skin prick test. For example, if you have extensive eczema or cannot stop taking antihistamines.

It is important to identify just a few key allergens if you are going to have one of these tests. To help your general practice team with this, keep a diary of your symptoms for several weeks.

Patch testing

This is different to skin prick testing and is usually done by a dermatologist. It is used to test for allergens that cause allergic contact dermatitis. Allergic contact dermatitis is a local skin inflammation that happens when you come in contact with something.

You can find out more information on patch testing on DermNet NZ.

Limitations of IgE testing by skin prick and EAST testing

IgE tests are not perfect tests. You will sometimes get a reaction when tested but it does not always mean you are allergic to that substance. This type of result is called a false positive.

Examples of when you might get a false positive result:

• You may get a reaction to something that was in the extract rather than the substance being tested. For example, if you have an allergy to birch pollen you might also have a reaction to peanut or tree nuts when tested on the skin prick testing. This is because there are proteins in the extract of the peanut test that are also in the birch pollen test.
• You may always have a high level of total IgE due to eczema, which can cause a greater reaction.
• IgE allergy testing cannot distinguish between sensitisation (when IgE is present, but you do not have any symptoms when in contact with the substance) and an allergy.

  HealthInfo recommends the following pages

• Allergy New Zealand

  Information and resources for people with allergies.

• Australasian Society of Clinical Immunology and Allergy (ASCIA):
  • Allergy testing

    More information about the different forms of allergy testing.

  • Evidence-based versus non-evidence-based allergy tests and treatments

    Information about alternative allergy testing methods.

Written by HealthInfo clinical advisers. Last reviewed July 2022.

Sources

Page reference: 85682

Review key: HIALL-38559