Skin Test For Allergy
What is a skin test for allergy?
This is a test done on the skin to identify the allergy substance
(the allergen) that is the trigger for an allergic reaction.
How is an allergy skin test done?
A small amount of the suspected allergy-provoking substance (the
allergen) is placed on the skin. The skin is then gently scratched
through the small drop with a special sterile needle. An allergy skin
test is also called a scratch test.
What is a positive skin test?
If the skin reddens and, more importantly, if it swells, then the
test is read as positive and allergy to that substance is considered
probable.
Can you give an example of a skin test?
If a specific food allergy is suspected, a skin test uses a dilute
extract of the suspect food. A small drop of this particular liquid
extract is placed on the skin of the forearm or back. This underlying
skin is gently scratched through the small drop with a special
sterile needle. If the skin reddens and, more importantly, if it
swells, then the test is read as positive. If there is no reaction,
it is read as negative.
If the skin test is positive, it implies that the patient has a
type of antibody (IgE antibody) on specialized cells in the skin that
release histamine to cause redness and itching. (These cells are
called mast cells and the IgE antibody bound to them is specific to
the food being tested.)
What are the advantages of skin tests?
Skin tests are rapid, simple, and relatively safe. They can be
very helpful in specifically identifying causes of allergies.
Is there danger to a skin test?
In some extremely allergic patients who have severe reactions
called anaphylactic reactions, skin testing cannot be used because it
could evoke a dangerous reaction. Skin testing also cannot be done on
patients with extensive eczema.
What is done if a skin test can't be done?
For these patients a doctor may use special blood tests, such as
the RAST and the ELISA. These tests measure the presence of specific
types of IgE in the blood.
These tests may cost more than skin tests, and results are not
available immediately. As with skin testing, positive RAST and ELISA
tests do not by themselves necessarily make the final diagnosis.
Last Editorial Review: 4/16/2002
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