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:
prick test. This test is done by placing a drop of a solution containing a possible
allergen on the skin, and a series of scratches or needle pricks allows the solution
to enter the skin. If the skin develops a red, raised itchy area (called a wheal),
it usually means that the person is allergic to that allergen. This is called a positive
Intradermal test. During this test, a small amount of the
allergen solution is injected into the skin. An intradermal allergy test may be done
when a substance does not cause a reaction in the skin prick test but is still suspected
as an allergen for that person. The intradermal test is more sensitive than the skin
prick test but is more often positive in people who do not have symptoms to that allergen
(false-positive test results).
patch test. For a skin patch test, the allergen solution is placed on a pad that
is taped to the skin for 24 to 72 hours. This test is used to detect a skin allergy
called contact dermatitis.
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.
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 or asthma.
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.
Why It Is Done
testing is done to find out what substances (allergens) may cause an allergic reaction.
The skin prick test can also be done to:
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
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
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
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) to2 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).
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.
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 hour each.
Skin patch test
skin patch test also uses small doses of the suspected allergen. For this test:
of allergens are placed on patches that look like adhesive bandages.
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 reaction.
The health professional drawing your blood will:
an elastic band around your upper arm to stop the flow of blood. This makes the veins
below the band larger so it is easier to put a needle into the vein.
the needle site with alcohol.
Put the needle into the vein. More than one
needle stick may be needed.
Attach a tube to the needle to fill it with blood.
the band from your arm when enough blood is collected.
Apply a gauze pad or
cotton ball over the needle site as the needle is removed.
to the site and then a bandage.
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 certain allergen.
How It Feels
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.
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.
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.
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.
You may develop a small bruise at the puncture site. You can
reduce the risk of bruising by keeping pressure on the site for several minutes after
the needle is withdrawn.
In rare cases, the vein may become inflamed after
the blood sample is taken. This condition is called phlebitis and is usually treated
with a warm compress applied several times daily.
or blood test can tell you what substance, or allergen, may trigger an allergic response.
Skin tests work by exposing a person
to suspected allergens and seeing if a reaction occurs. The results of the skin tests
are 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.
of immunoglobulin E (IgE), a type of antibody, are the same as in a person who does
not have allergies.
levels of immunoglobulin E (IgE) antibodies for a particular allergen or group of
allergens are above the normal level.
What Affects the
Reasons you may not be able to have a skin test or why the results may
not be helpful include:
Exercise that causes excessive sweating.
a skin patch wet.
Taking medicines such as antihistamines or tricyclic antidepressants.
To Think About
You may not need allergy testing if you have mild allergies
that are easily controlled with medicine or lifestyle changes.
Are the easiest and least expensive method for identifying
allergies in most people.
Are more reliable than blood tests for identifying
common inhaled (airborne) allergies, such as pollens, dust, mold, and pet dander.
tell whether a person is reacting to a certain food. Further testing (such as an elimination
diet or an oral food challenge) may be needed to identify this. In an elimination
diet, you avoid eating foods that may be causing an allergic reaction. In an oral
food allergy challenge, you eat a variety of foods that may or may not cause an allergic
reaction. Your doctor watches to see if and when a reaction occurs.
unpleasant for children and their parents.
Blood tests for allergies:
Are less sensitive than skin tests. So it is possible for people to have a
positive reaction to a skin test but have a negative reaction to a blood test.
not affected by antihistamines or tricyclic antidepressants, so people do not need
to stop taking these medicines while being tested.
May be a good choice if
you have very sensitive skin or a skin problem, such as eczema, that would make it
hard to see whether you have a reaction to a skin test.
Will not cause an
allergic reaction. They may be used if you have had a serious allergic reaction in
Cost more than skin tests.
You may need to be
tested for a latex allergy before having a major surgery if you have had frequent
exposure to latex. Allergy blood tests are used for this. If you have a latex allergy,
latex products should not be used during your surgery or recovery.
tests, such as fluoro-allergosorbent testing (FAST), multiple antigen simultaneous
testing (MAST), and radioallergosorbent testing (RAST), are sometimes used to check
Pagana KD, Pagana TJ (2010). Mosby's
Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
ByHealthwise Staff Primary
Medical ReviewerE. Gregory Thompson, MD - Internal
Medicine Adam Husney, MD - Family Medicine Martin J. Gabica, MD - Family Medicine Kathleen
Romito, MD - Family Medicine Rohit K. Katial, MD
- Allergy and Immunology
This information does not replace the advice of a doctor.
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