Allergy Tests

How It Is Done

Skin tests

The health professional doing the skin prick or intradermal test will:

• Clean the test site (usually on your back or arm) with alcohol.
• Place drops of the possible allergens on your skin about 1 in (2.5 cm) to 2 in (5 cm) apart. This allows many substances to be tested at the same time.
• Prick the skin under each drop with a needle. The needle passes through the drop and allows some of the allergen to penetrate your skin. For the intradermal test, a needle is used to inject the allergen solution deeper into the skin. See a picture of a skin prick allergy test .
• Check the skin after 12 to 15 minutes for red, raised itchy areas called wheals. If a wheal forms, it means you are allergic to that allergen (this is called a positive reaction).

An alternative skin prick method uses a device with 5 to 10 points (heads), which are dipped into bottles that contain the allergen extract. This device is pressed against the skin of the forearm or back so that all heads are pressed into the skin at the same time.

If the skin prick test is negative, you may choose to have an intradermal skin test at a later visit. A skin prick test is usually done first because the intradermal test has a greater chance of causing a severe allergic reaction.

The skin prick test and the intradermal test usually take less than an hour each.

A skin patch test also uses small doses of the suspected allergen. For this test:

• Doses of allergens are placed on patches that look like adhesive bandages.
• The patches are then placed on the skin (usually on your back). This usually takes about 40 minutes, depending on how many patches are applied. See a picture of a skin patch allergy test .
• You will wear the patches for 24 to 72 hours. Do not take a bath or shower or do any activities that could make you sweat excessively while you are wearing the patches. This could loosen the patches and cause them to fall off.
• The patches will be removed by your health professional, and your skin will be checked for signs of an allergic reaction.

Blood test

The health professional drawing your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Apply a gauze pad or cotton ball over the needle site as the needle is removed.
• Apply pressure to the site and then a bandage.

The blood sample will be placed on specially treated paper and sent to a lab to determine whether antibodies to any of the allergens being tested are present. If specific antibodies are found, it may mean you are allergic to a certain allergen.

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Author:	Bets Davis, MFA	Last Updated: July 2, 2009
Medical Review:	E. Gregory Thompson, MD - Internal Medicine Harold S. Nelson, MD - Allergy and Immunology
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