Test Overview
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.
Skin tests
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:
- Skin 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 reaction.
- 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).
- Skin 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.
Blood test
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
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. For more information, see:
Allergies: Should I have allergy shots?
Why It Is Done
Allergy testing is done to find out what
substances (allergens) cause an allergic reaction.
Skin test
The skin prick test can also be done
to:
- Identify inhaled (airborne) allergens, such
as tree, shrub, and weed pollens, moulds, dust, feathers, and pet
dander.
- Identify likely food allergens (such as eggs, milk,
peanuts, nuts, fish, soy, wheat, or shellfish).
- Determine whether
a person may be allergic to a
medicine or insect venom.
Blood test
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
the substance.
- Has had a severe allergic reaction (anaphylaxis).
- Has had positive skin tests
to many foods. Enzyme-linked immunosorbent assay (ELISA) can find out the foods
that a person is most allergic to.
How To Prepare
Many medicines can affect the results of
a skin test. Be sure to tell your health professional about all the
non-prescription and prescription medicines you take.
You may need to stop taking some medicines, such as some tricyclic
antidepressants and
antihistamines such as cetirizine (Reactine),
fexofenadine (Allegra), and loratadine (Claritin) before you have an allergy
skin test.
Talk to your health professional 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
Skin tests
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 possible allergens on your
skin about 2.5 cm (1 in.) to
5 cm (2 in.) 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. See a picture of a
skin prick allergy test.
- 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
hour each.
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. See a picture of a
skin patch allergy test.
- 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.
Blood test
The health professional drawing your
blood will:
- Wrap 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.
- Clean 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.
- Remove 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.
- Apply pressure 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 detected, it may mean you are allergic to a
certain allergen.
How It Feels
Skin tests
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 non-prescription
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 health professional.
Blood test
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.
Risks
Skin tests
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, difficulty
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 health professional.
Blood test
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.
- Continued bleeding can be a problem
for people with bleeding disorders. ASA (Aspirin), warfarin (Coumadin) and
other blood-thinning medicines can also make bleeding more likely. If you have
bleeding or clotting problems, or if you take blood-thinning medicine, tell
your health professional before your blood is drawn.
Results
A skin or blood test can tell you what
substance, or
allergen, may trigger an
allergic response.
Skin tests
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.
Allergy skin tests Normal (negative): | No raised red areas (called wheals) are created by the
allergen. |
|---|
Abnormal (positive): | A wheal created by the allergen is at least 3 mm (1/8
inch) 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.
See a picture of a
positive patch test reaction. |
|---|
Blood tests
Allergy blood tests look for substances
in the blood called antibodies. Results of allergy blood tests are usually
available in about 7 days.
Allergy blood tests Normal (negative): | The levels of immunoglobulin E (IgE), a type of
antibody, are the same as in a person who does not
have allergies. |
|---|
Abnormal (positive): | The levels of immunoglobulin E (IgE) antibodies for a
particular allergen or group of allergens are 4 times the normal level.
|
|---|
What Affects the Test
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.
- Getting a skin patch wet.
- Taking medicines
such as
antihistamines or tricyclic antidepressants.
What To Think About
- You may not need allergy testing if you have
mild allergies that are easily controlled with medicine or lifestyle
changes.
- Skin tests:
- 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, mould, and pet dander.
- Cannot tell whether a person
is reacting to a certain food. Further testing (such as an elimination diet)
may be needed to identify this.
- Can be unpleasant for children and
their parents.
- May take as long as 3 to 4 hours.
- Allergy blood tests:
- Are less sensitive than skin tests.
Therefore, it is possible for people to have a positive reaction to a skin test
but have a negative reaction to a blood test.
- Are 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 the
past.
- 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.
- Other
blood tests, such as fluoro-allergosorbent testing (FAST), multiple antigen
simultaneous testing (MAST), and radioallergosorbent testing (RAST), are
sometimes used to check for allergens.
References
Other Works Consulted
- Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed.
Philadelphia: Saunders.
- Fischbach FT, Dunning MB III, eds. (2004).
Manual of Laboratory and Diagnostic Tests, 7th ed.
Philadelphia: Lippincott Williams and Wilkins.
- Handbook of Diagnostic Tests
(2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.
Credits
| By | Healthwise Staff |
|---|
| Primary Medical Reviewer | E. Gregory Thompson, MD - Internal Medicine |
|---|
| Specialist Medical Reviewer | Harold S. Nelson, MD - Allergy and Immunology |
|---|
| Specialist Medical Reviewer | Brian D. O'Brien, MD - Internal Medicine |
|---|
| Last Revised | February 1, 2010 |
|---|
