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
antihistamines such as cetirizine (Zyrtec),
fexofenadine (Allegra), and loratadine (Claritin), before you have an allergy
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 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
Skin patch test
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.
- 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