Open a PDF version to print this topic

HealthInfo Canterbury

Allergy testing

Allergy testingAllergy 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 GP 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

Canterbury Health Laboratories runs an allergy skin prick testing clinic. You'll need to be referred to this clinic by your GP. This clinic often has long waiting times (six or more weeks). Children are given priority because getting a blood test from them can be more difficult. Some doctors may perform skin prick testing privately.

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 wheal, 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 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 wheal, it means your immune system is sensitised to the allergen. In young children the larger the wheal, the more likely a food allergy may exist.

In adults, a bigger wheal doesn’t tell you if you're more likely to be allergic or tell you if you will 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 the substance. This happens when you have a sensitivity to the substance but don't have an allergy to it.

Skin prick testing is useful for:

Skin prick testing is not useful for:

RAST/EAST 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, but not both.

EAST 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 GP 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 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 are not 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:

  HealthInfo recommends the following pages

Written by HealthInfo clinical advisers. Last reviewed July 2018.

Sources

Page reference: 85682

Review key: HIALT-85682