Allergy testing Whakamātaunga mate pāwera
Different types of test are used to check for allergies. These include skin prick testing, EAST/RAST (blood) testing and patch testing. The test used depends on what type of allergy you or your healthcare provider thinks you may have.
What an allergy is
Allergy is also called a hypersensitivity reaction. It is a type of reaction that can happen in your body when you are exposed to a foreign substance, also called an allergen or sometimes a trigger.
Allergic reactions usually happen within an hour of you being exposed to an allergen. Allergens can include:
- something in the environment, like pollen or dust mites
- a food
- a medication.
Allergies are controlled (mediated) by proteins in your immune system. The most common types of allergies are controlled by proteins called IgE antibodies.
The allergy tests described here look for the types of reaction involving IgE antibodies. These are often called IgE-mediated allergies.
Other hypersensitivity reactions
There are many other types of hypersensitivity, including:
- chemical sensitivity (such as reactions to household cleaners)
- food intolerances (such as lactose or gluten).
These types of hypersensitivity reactions do not involve the immune system and are not controlled (mediated) by IgE. They are called non-IgE mediated hypersensitivity.
Another difference between sensitivities, intolerances and allergies is that:
- with allergies it may take only a very tiny amount of an allergen to trigger a severe reaction
- with intolerances and sensitivities, these usually depend on the amount — you only have a more severe reaction if you are exposed to a larger amount of the substance you are intolerant or sensitive to.
Your healthcare provider will look at your health record to try to find out if the reaction is an IgE-mediated allergy or a non-IgE mediated hypersensitivity. You can help by keeping a diary of your symptoms and what you think might trigger (cause) them.
Types of allergy tests
Allergy testing is only helpful to confirm strongly suspected specific allergies, such as a peanut allergy. Testing cannot include every substance (allergen) you might be allergic to, so it is important to know what you think you are reacting to.
Skin prick testing
Skin prick testing looks at how your skin responds to certain allergens. A liquid containing the allergen (such as cat fur) is placed on your skin. 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 (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.
This test will not work properly if you are taking antihistamines, a common treatment for allergies. Stop taking all antihistamines at least 72 hours before your test.
The test is also not reliable if done within 4 weeks of severe allergic reaction (anaphylaxis) or an acute (sudden onset) allergic reaction to food.
Skin prick testing is useful for:
- some cases of acute food reaction causing anaphylaxis, hives (urticaria) or facial swelling (angioedema) within 30 to 60 minutes of being exposed to that food
- 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
- long-term (chronic) hives
- eczema (unless there is urticaria as well)
- allergic contact dermatitis
- food intolerances including bloating, runny poos (diarrhoea) and extreme tiredness (fatigue)
headaches - behavioural problems.
EAST/RAST blood test
EAST/RAST is a blood test that looks for IgE antibodies to specific allergens. If it finds them, it means your immune system is sensitised (and so reacts) to those allergens.
It is looking for the same thing as skin prick testing. You likely only need to do one test or the other, not both.
EAST/RAST blood tests are useful when you cannot do a skin prick test, for example if you:
- have a lot of eczema, so there is no clear skin area to use
- cannot stop taking antihistamines.
It is important to focus on just a few key allergens if you are going to have one of these tests. To help your healthcare provider with this, keep a diary of your symptoms for several weeks.
Limitations of skin prick and EAST/RAST testing
Skin prick and EAST/RAST testing are not perfect for testing IgE. 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 — it can happen for different reasons.
- These tests use extracts of the things you might be allergic to. You may get a reaction to something else in the test extract rather than the actual allergen being tested.
- Some allergens can be in more than one test extract.
- You may always have a high level of IgE due to having eczema or another medical condition — this can cause a greater reaction.
- IgE allergy testing cannot tell between sensitisation (when IgE is present, but you do not have any symptoms when in contact with the substance) and an allergy.
Patch testing
Patch testing is different to skin prick testing. It is usually done by a skin specialist (dermatologist) or allergy specialist.
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 about patch testing at DermNet.