Why It Is Done
Allergy testing is done to find out what substances (allergens) may cause an allergic reaction.
The skin prick test can also be done to:
- Identify inhaled (airborne) allergens, such as tree, shrub, and weed pollens, molds, dust, feathers, and pet dander.
- Identify likely food allergens (such as eggs, milk, peanuts, nuts, fish, soy, wheat, or shellfish).
- Find out whether a person may have a drug allergy or be allergic to insect venom.
A blood test on a blood sample may be done instead of a skin prick test if a person:
- Has hives or another skin condition, such as eczema, that makes it hard to see the results of skin testing.
- Cannot stop taking a medicine, such as an antihistamine or tricyclic antidepressant, that may prevent or reduce a reaction to a substance even when a person is allergic to the substance.
- Has had a severe allergic reaction (anaphylaxis).
- Has had positive skin tests to many foods. Enzyme-linked immunosorbent assay (ELISA) can find out the foods that a person is most likely allergic to.
How To Prepare
Many medicines can affect the results of a skin test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. You may need to stop taking some medicines, such as some tricyclic antidepressants and antihistamines such as cetirizine (Zyrtec), fexofenadine (Allegra), and loratadine (Claritin), before you have an allergy skin test.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will show. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).
How It Is Done
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 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.
- Check the skin after 12 to 15 minutes for red, raised itchy areas called wheals. If a wheal forms, it means you are possibly allergic to that allergen (this is called a positive reaction).