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:
- 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
- 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 .
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.
Why It Is Done
Allergy testing is done to find out what
substances (allergens) cause an allergic reaction.
The skin prick test can also be done
- Identify inhaled (airborne) allergens, such
as tree, shrub, and weed pollens, molds, dust, feathers, and pet
- 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.
A blood test on a blood sample may be
done instead of a skin prick test if a person:
hives or another skin condition, such as
eczema, that makes it hard to see the results of skin
- 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 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.