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.
Allergy skin tests
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.
Allergy blood tests
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.
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
- Getting a skin patch wet.
- Taking medicines
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
- Skin tests for allergies:
- 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.
- 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
- May take as long as 3 to 4 hours.
- 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.
- Are not affected by
antihistamines or tricyclic antidepressants, so people
do not need to stop taking these medicines while being tested.
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
- 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.
blood tests, such as fluoro-allergosorbent testing (FAST), multiple antigen
simultaneous testing (MAST), and radioallergosorbent testing (RAST), are
sometimes used to check for allergens.