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Allergy testing involves having a skin or
blood test to find out what substance, or
allergen, may trigger an
allergic response in a person. Skin tests are usually
done because they are rapid, reliable, and generally less expensive than blood
tests, but either type of test may be used.
A small amount of a suspected allergen
is placed on or below the skin to see if a reaction develops. There are three
types of skin tests:
Allergy blood tests look for
substances in the blood called
antibodies. Blood tests are not as sensitive as skin
tests but are often used for people who are not able to have skin tests.
The most common type of blood test used is the enzyme-linked
immunosorbent assay (ELISA, EIA). It measures the blood level of a type of
antibody (called immunoglobulin E, or IgE) that the body may make in response
to certain allergens. IgE levels are often higher in people who have allergies
Other lab testing methods, such as radioallergosorbent
testing (RAST) or an immunoassay capture test (ImmunoCAP, UniCAP, or Pharmacia
CAP), may be used to provide more information.
Your allergy test results may show that allergy
treatment is a choice for you.
Allergy testing is done to find out what
substances (allergens) cause an allergic reaction.
The skin prick test can also be done
A blood test on a blood sample may be
done instead of a skin prick test if a person:
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?).
The health professional doing the
skin prick or intradermal test will:
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
A skin patch test also uses
small doses of the suspected allergen. For this test:
The health professional drawing your
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
With the skin prick test and the
intradermal skin test, you may feel a slight pricking sensation when the skin
beneath each sample is pricked or when the needle penetrates your skin.
If you have an allergic reaction from any of the skin tests, you may have
some itching, tenderness, and swelling where the allergen solutions were placed
on the skin. After the testing is done, cool cloths or a nonprescription
steroid cream can be used to relieve the itching and swelling.
you are having a skin patch test and you have severe itching or pain under any
of the patches, remove the patches and call your doctor.
You may feel nothing at all from the
needle puncture, or you may feel a brief sting or pinch as the needle goes
through the skin. Some people feel a stinging pain while the needle is in the
vein. But many people do not feel any pain or have only minor discomfort after
the needle is positioned in the vein.
The major risk with the skin prick test
or the intradermal skin test is a severe allergic reaction called
anaphylaxis. Symptoms of a severe allergic reaction
include itching, wheezing, swelling of the face or entire body, trouble
breathing, and low blood pressure that can lead to
shock. An anaphylactic reaction can be
life-threatening and is a medical emergency. Emergency care is always needed
for an anaphylactic reaction. But severe allergic reaction is rare, especially
with the skin prick test.
If you are having a skin patch test and
you have severe itching or pain under any of the patches, remove the patches
and call your doctor.
There is very little risk of a problem
from having blood drawn from a vein.
A skin or blood test can tell you what
allergen, may trigger an
Skin tests work by exposing a person
to suspected allergens and seeing if a reaction occurs. The results of the skin
tests will be available immediately after testing is done.
No raised red areas (called wheals) are created by the
A wheal created by the allergen is at least 1/8 inch
(3 mm) larger than the reaction to the negative control. The larger the wheal,
the more certain it is that the person is allergic to that specific allergen.
Allergy blood tests look for
substances in the blood called antibodies. Results of allergy blood tests are
usually available in about 7 days.
The levels of immunoglobulin E (IgE), a type of
antibody, are the same as in a person who does not
The levels of immunoglobulin E (IgE) antibodies for a
particular allergen or group of allergens are 4 times the normal level.
Reasons you may not be able to
have a skin test or why the results may not be helpful include:
Other Works Consulted
Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
June 30, 2011
E. Gregory Thompson, MD - Internal Medicine & Rohit K Katial, MD - Allergy and Immunology
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