HealthInfo 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 aren't controlled by IgE and 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's 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'll develop a red bump, or weal within 10 to 20 minutes.

The test won't work properly if you're taking antihistamines. You'll need to stop all antihistamines at least 72 hours before your test. The test also isn't reliable if performed within four weeks of anaphylaxis or an acute allergic reaction to food.

If you have no reaction, it means you're unlikely to be allergic to that substance. But if you get a red bump, or weal it means your immune system is sensitised to the allergen. In young children, the larger the weal, the more likely they have a food allergy.

In adults, a bigger weal doesn’t tell you if you're more likely to be allergic or if you'll react to this in the future. There's also a possibility that you can have a skin prick reaction to a substance but don't have symptoms when you're normally exposed to it. This happens when you have a sensitivity to the substance but don't have an allergy to it.

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's 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 can't do a skin prick test. For example, if you have extensive eczema or can't stop taking antihistamines.

It's 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's used to test for allergens that cause allergic contact dermatitis (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 aren't perfect tests. You'll sometimes get a reaction when tested but it doesn't always mean that you're 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 can't tell sensitisation (when IgE is present, but you don't have any symptoms when in contact with the substance) apart from 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