Description of the test

An allergy skin test identifies which substance triggers an allergic reaction. A small amount of the substance, or allergen, is introduced to the body through the skin. If the skin reddens or swells then the test is read as positive and the person is probably allergic to that substance. The reaction is due to the release of histamine, a chemical naturally found in the body, which causes swelling, redness, and itching (also known as a wheal). This test is performed by an allergist.

How often should the test be performed?

Your doctor will decide when and how frequently this test is required.

Why is this test performed?

Allergy skin tests are performed to identify what allergen is causing allergy symptoms (such as sneezing, nasal congestion, itchy eyes, wheezing, skin rash, and swelling) so that people can avoid the trigger if necessary. There are many allergens - including foods, medications, and environmental substances (such as ragweed and fungus), as well as contact allergens which irritate the skin.

Allergy skin tests take place when allergy shots are being considered. Testing may also be necessary for people with serious allergic rhinitis or asthma.

Are there any risks and precautions?

Some people have extremely serious allergic reactions, called anaphylactic reactions. If a person has had an anaphylactic reaction to an allergen in the past, they may not have an allergy skin test for this substance as the test might provoke a dangerous reaction.

The most serious risk of having a skin prick test is anaphylactic reaction. This reaction is a medical emergency, causing difficulty breathing and a dangerously low blood pressure. However, anaphylactic reactions with skin prick tests are rare and the allergist will be monitoring you closely.

Skin testing may not be performed if you have certain skin conditions, such as severe eczema or atopic dermatitis.

What happens during the test?

There are three different types of skin tests.

The first is called a skin prick test or scratch test. A small drop of substance is placed on the top of the skin. Then, the skin underneath the drop is pricked or scratched with a sterile needle to introduce the allergen to the top layers of the skin. If the area becomes red or swells, the allergy skin test is positive. If there is no reaction, the test is negative.

The second test is called an intradermal test. A small amount of substance is injected into the skin to reach deeper layers of the skin. This test shows whether a person has antibodies to the allergen. The intradermal test is done when the skin prick test is negative but the patient is still thought to be allergic to that substance.

The third test is the skin patch test. The substance is placed on a patch and the patch is placed on the skin for up to two days. This test shows whether a person develops contact dermatitis (rash), which is an allergic reaction on the skin due to direct contact with an allergen.

Allergy skin tests are typically done on the back or the back of the arm, depending on the number of allergens being tested. If a person is being tested for several allergens, a grid may be used to separate the different substances from each other and keep track of them.

A reaction can usually be seen within 15 minutes.

The doctor may choose to do a blood test, called the radioallergosorbent test (RAST), with the skin allergy test. This test measures the levels of antibodies in the blood. The levels of antibodies may be elevated if a person is allergic to the substance. The RAST can also be done if a person cannot have a skin test.

How should I prepare for this test?

There are no food or drink restrictions before going for an allergy skin test or a RAST. Your doctor may ask you to stop taking some medications before the tests, such as antidepressants or antihistamines which may affect the allergy skin test results. Antidepressants and antihistamines do not have any affect on a RAST.

Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications that you are taking. Also tell them about any medication allergies and medical conditions that you may have.

Ask your doctor or pharmacist whether you need to stop taking any of your medications before the test.

What can I expect after the test?

If your allergy skin test is positive, you have several options to prevent an allergic reaction from occurring again. Some people choose to undergo desensitization, where small amounts of the allergen are gradually introduced to the body so that the person develops immunity to the allergen.

If an allergic reaction develops, antihistamines can be used to relieve symptoms.

Your allergist may also discuss helpful strategies to help you avoid or minimize exposure to allergens.

Results

The skin prick test and intradermal test can be read within about 15 minutes and the allergist can tell you the result during that same visit. The skin patch test requires you to return to your doctor's office after having worn the patch for two days. At that point, your doctor can tell you the results. RAST samples are sent to a lab and may take one to two weeks before results are available.