Allergies: Skin testing for allergies

An allergy skin test is used to identify the substances that are causing your allergy symptoms. It is often performed by applying an extract of an allergen to your skin, scratching or pricking the skin to allow exposure, and then evaluating the skin’s reaction. It may also be done by injecting the allergen under the skin, or by applying it to a patch that is worn on the skin for a specified period of time.

What happens during the test?

The procedure varies depending on what type of skin test you are having. The three main types of skin tests are the scratch test, intradermal test, and the patch test.

• Scratch test (also known as a puncture or prick test): first, a doctor or nurse will examine the skin on your forearm and clean it with alcohol. (Sometimes, the skin test is performed on an area of your back.) Areas on your skin are then marked with a pen to identify each allergen that will be tested. A drop of extract for each potential allergen - such as pollen, animal dander or insect venom ­- is placed on the corresponding mark. A small disposable pricking device is then used so the extract can enter into the outer layer of the skin, called the epidermis. The skin prick is not an injection and does not cause bleeding.
• Intradermal test: after examining and cleaning the skin, a small amount of the allergen is injected just under the skin.
• Patch test: another method is to apply an allergen to a patch, which is then placed on the skin. This may be done to pinpoint a trigger of allergic contact dermatitis. If there are allergic antibodies in your system, your skin will become irritated and may itch, much like a mosquito bite. This reaction would mean you are allergic to that substance.

How long does the test take?

Plan to spend about an hour for the entire appointment. For the scratch and intradermal skin tests, the allergen placement part of the test takes about five-10 minutes. Then you will have to wait about 15 minutes to see how your skin reacts.

Patch tests take longer, as you must wear the patch for about 48 hours to determine if there is a delayed reaction. During this time, bathing and swimming should be avoided. You will return to the doctor after the specified period of time to have the area(s) checked.

How should I prepare for the test?

Tell the healthcare provider who will perform the skin test about any medicines you are taking, including over-the-counter medications.

Since allergy medicines, including over-the-counter antihistamines, stop allergic reactions, you should not take them for a few days before the test. Talk to your doctor about discontinuing your allergy medicines prior to the test.

Your doctor will give you a list of medicines to avoid before the test, since there are other drugs that can interfere with the results. If you cannot discontinue certain medicines, the doctor or nurse may perform a separate ‘control’ test to determine if that particular drug will interfere with the scratch test.