How do you confirm an allergic reaction?

During skin prick testing, an allergy doctor (allergist) or nurse puts a tiny bit of a liquid containing an allergen into the skin by making a small scratch or prick on the skin. Allergists usually do skin testing on a person's forearm or back. The allergist then waits 15 minutes or so to see if a red, raised bump (called a wheal) forms. If it does, there might be an allergy. The allergist will use a ruler to measure the wheal and the redness around it.

If someone might be allergic to more than one thing — or if it's not clear what's triggering a person's allergy symptoms — the allergist may skin test for several different allergens at the same time.

Skin tests may itch for a little while, but this usually gets better after about 30 minutes. If your child is itchy after the test, the allergist might give them an antihistamine to take or apply a topical steroid cream to your child’s back or arm to ease the itching.

A skin test that shows up as positive to a food only means a person might be allergic to that food. The allergist may request a blood test as well, sending a small blood sample to a lab for analysis. The lab checks the blood for IgE to specific foods. Again, if there are IgE antibodies to a food, it means the person might be allergic to it.

If the results of the skin test and blood test are unclear, an allergist might do something called an oral food challenge. During an oral food challenge, a person slowly eats increasing amounts of the potential food allergen while the doctor watches for symptoms.

Because food allergies can trigger serious reactions in people, this test must be done in an allergist's office or a hospital that has access to medicines and specialists to control these reactions. Most of the time, this type of test is done to find out if someone has outgrown a known food allergy.

What Happens if a Test Shows an Allergy?

If your child does have an allergy, the allergist will work with you on a plan to keep your child safe. You’ll learn how to help your child avoid foods that aren’t safe. The doctor also will give you a prescription for emergency medicine (called epinephrine) to keep with your child all the time in case of a severe allergic reaction.

Avoiding or minimising allergic triggers is an important part of allergy and asthma management. Allergy testing using skin prick tests or blood tests for allergen-specific IgE helps your doctor to confirm the substances to which you are allergic, so that appropriate advice can be given.

When allergy testing is appropriate

Allergy testing is usually performed on people with suspected allergic rhinitis (hay fever), asthma or reactions to insects or foods. In people with allergic rhinitis or asthma, allergy testing usually includes house dust mite, cat and dog dander (or other animals if contact occurs), mould spores, pollen from relevant grasses, weeds or trees and in some cases, occupational allergens. Testing can also be used to confirm suspected allergies to foods, stinging insects and some medicines.

It is important to note that:

• Allergy test results cannot be used on their own and must be considered together with your clinical history.
• Medicare rebates are available for skin prick tests or blood tests for allergen specific IgE (formerly known as RAST) in Australia.
• In some cases, you may be referred to a clinical immunology/allergy specialist for further detailed assessment.

Skin prick testing

Skin prick testing is the most convenient method of allergy testing. It has been shown in clinical studies to improve the diagnosis of allergy. As results are available within 20 minutes, this allows you to discuss the outcome with your doctor at the time of testing.

Tests are most commonly performed on the forearm or the back. The skin is first cleaned with alcohol and may be marked with numbers corresponding to the allergens. Using a sterile lancet, a small prick is made through a drop of allergen extract. This allows a small amount of allergen to enter the skin.

If you are allergic to the tested allergen, a small itchy lump (wheal) surrounded by a red flare will appear within 15-20 minutes.

Skin prick tests are slightly uncomfortable but are usually well tolerated, even by small children. Local itch and swelling normally subside within one to two hours. More prolonged or severe swelling may be treated with a non-sedating antihistamine, a painkiller tablet and/or a cool compress. Occasionally people will feel dizzy or light-headed and need to lie down. Severe allergic reactions from allergy testing for asthma or allergic rhinitis (hay fever), are very rare.

Skin prick testing should only be performed by a health professional who has been trained in the procedure and who knows how to select the allergens, interpret the results and manage any generalised allergic reaction that may occur. Allergic reactions to skin prick testing are rare.

Antihistamine tablets/syrups or medications with antihistamine-like actions (such as some cold remedies and antidepressants) should not be taken for three to seven days before testing as these will interfere with the results. You may also be advised to avoid creams and moisturisers on your forearms or back on the day of the test, to reduce the likelihood that allergen extracts will run into each other.

Skin prick testing has no value in the investigation of suspected reactions to aspirin, food additives, or respiratory irritants like smoke or perfumes.

Other skin testing methods

Intradermal testing (also known as scratch testing) should not be used to test for allergy to inhalants or foods. Instradermal testing was used in the past, however it is less reliable than skin prick testing, and causes much greater discomfort. Intradermal skin testing may be used to test for allergies to antibiotic drugs or stinging insect venom, when greater sensitivity is needed. 

Blood tests for allergen specific IgE

Immunoglobulin E (IgE) antibodies directed against specific allergens can be measured with a blood test. These tests are often performed when skin testing is not easily available, when there is a skin condition such as severe eczema, or when a person is taking medications (such as antihistamines), that interfere with accurate skin prick testing.

Total IgE testing

Total IgE antibodies may be raised in people with allergies, and can be measured from a blood sample.  High total IgE levels are also found in people with parasite infections, eczema and some rare medical conditions. High total IgE levels do not prove that symptoms are due to allergy, and a normal total IgE level does not exclude allergy. Therefore, measuring total IgE levels is not routinely recommended in allergy testing.

Eosinophil counts

Eosinophils are specialised white blood cells that are designed to kill worms and parasites. They can also cause tissue inflammation in allergy. High levels are sometimes seen in blood samples from people with allergic rhinitis (hay fever), asthma and eczema, as well as other less common conditions. However, a high eosinophil count does not prove that symptoms are due to allergy, and a normal eosinophil count does not exclude allergy. Therefore measuring eosinophil counts has a limited role to play in allergy testing.

Patch testing

Patch testing is useful for testing for allergic contact dermatitis, that can be triggered by metal, cosmetic preservatives or various plants. It involves applying patches with test substances in small chambers or discs to a person's back, which are secured with hypoallergenic tape. The tapes are left in place for 48 hours and kept dry for the entire time. The test site is then read at different time intervals. An eczema-like rash can indicate sensitivity to a particular allergen.

Oral allergen challenge testing

Oral allergen challenge testing may sometimes be required to confirm diagnosis when the cause of a severe allergic reaction has not been confirmed. This will normally only be performed using foods or medications under the supervision of a clinical immunology/allergy specialist with appropriate resuscitation facilities immediately available.

Unproven methods

There are several methods that claim to test for allergy and many of these are offered as online allergy tests, which ASCIA does NOT recommend. Unproven methods include cytotoxic food testing, kinesiology, Vega testing, electrodermal testing, pulse testing, reflexology and hair analysis. 

These methods have not been scientifically validated and can result in misdiagnosis and ineffective treatments, leading to unnecessary, costly and (in the case of some changes in diet), dangerous avoidance strategies. No Medicare rebate is available in Australia for these tests and the use of these methods is not recommended.

© ASCIA 2020

ASCIA is the peak professional body of clinical immunology/allergy specialists in Australia and New Zealand.

ASCIA resources are based on published literature and expert review, however, they are not intended to replace medical advice.

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