Scratch Test
for Allergies
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What is the
test?
This test checks for a skin reaction to common
allergy-provoking substances, such as foods,
molds, dust, plants, or animal proteins. If your
skin reacts to a substance, chances are that
you are allergic to it.
Most people with allergy symptoms don’t
need testing because they can identify their
triggers and control their symptoms with medicine.
Your doctor might recommend scratch testing when
you have severe allergy symptoms but are not
sure what is causing them. Knowing what you are
allergic to can help you avoid the substance
in the future, and will help your doctor determine
whether you might benefit from allergy shots.
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How
do I prepare for the test?
Don’t take antihistamines for three days
before the test, because they can interfere with
the results by preventing you from reacting to
a substance you would normally be allergic to.
Tell your doctor if you have ever had anaphylaxis,
a lifethreatening allergic reaction, or if you
have had a serious reaction to a previous allergy
test.
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What
happens when the test is performed?
In adults, the test is done on the forearm;
in children it’s done on the upper back.
(The child disrobes from the waist up and lies
on his or her stomach.)
Your doctor decides what allergies are to be
tested for. Some people are tested for as many
as a few dozen at one visit. Individual drops
of fluid are dripped in rows across the skin.
The doctor uses a needle to make small light
scratches in the skin under each drop, to help
the skin absorb the fluid. The scratches aren’t
deep enough to cause bleeding. Each drop contains
proteins from a separate allergen (a substance,
like ragweed pollen, that triggers allergy symptoms).Your
doctor notes where each drop of fluid was placed,
either by keeping a chart or by writing a code
on the area of skin being tested.
For many people, the most difficult part of
this test is next: You need to stay still long
enough (usually about 20 minutes) to give the
skin time to react. Your skin might tickle or
itch during this time, but you won’t be
allowed to scratch it. At the end of the waiting
time, your doctor will examine each needle scratch
for redness or swelling.
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What
risks are there from the test?
Because the allergen exposure is so small, a
serious allergic reaction is extremely unlikely.
For a few hours you’ll probably have some
redness or irritation on the testing sites, similar
to having several mosquito bites.
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Must
I do anything special after the test is over?
No.
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How
long is it before the result of the test is
known?
Your doctor can tell you right away which substances
caused a reaction. The next step is to see if
you can prevent an allergic reaction by avoiding
those substances, or relieve symptoms by taking
medicine or allergy shots.
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