All about
allergic test
Why the
test Is performed?
Allergy tests are
used to determine the specific substances that cause an allergic
reaction in an individual. They may also be used to determine if a group
of symptoms is a true allergic reaction (involving antibodies and
histamine release).
How to
perform the test?
There are many
methods of allergy testing. Among the more common are the skin tests,
elimination-type tests, and the radioallergosorbent test (RAST).
Skin tests are
the most common. Specific methods vary. The scratch test (one of the
most common methods) involves placement of a small amount of suspected
allergy-causing substances on the skin, and then scratching or pricking
the skin so that the allergen is introduced under the skin surface. The
skin is observed closely for signs of a reaction, usually swelling and
redness of the site. Results are usually obtained within about 20
minutes, and several suspected allergens can be tested at the same time.
Skin tests are most useful for respiratory allergies , a penicillin
allergy, and insect bite allergies.
Food allergies
are usually tested by using various "elimination" diets (in
which the suspected food(s) are eliminated from the diet for several
weeks and then gradually re-introduced one at a time while the person is
observed for signs of an allergic reaction). Because food allergies are
often affected by what the person thinks they might be allergic to, a
double-blind test may be advised.
The RAST is a
laboratory test performed on blood (see venipuncture). It tests for the
amount of specific IgE antibodies in the blood (which are present if
there is a "true" allergic reaction).
Preparation
for the test
Before any
allergy testing, the health care provider will ask for a very detailed
medical history. This may include questions about such things as
illnesses, emotional and social conditions, work, entertainment,
lifestyle, foods, and eating habits.
Abnormal
results`s meaning
Most often, a
positive test indicates an allergy to the substance in question. The
skin tests are most reliable when testing for airborne substances (such
as animal dander or pollen). However, if the dose of allergen is
excessive, a positive reaction will occur even in persons who are not
allergic.
The risks
of the test
Skin tests and
food allergy tests carry a risk that you will experience an allergic
reaction when exposed to the allergens. Some sensitive persons will
(rarely) experience a life-threatening anaphylactic reaction.
Penicillin are
the only drugs that can be tested using skin tests. (Skin tests for
other drugs are at best non-informative and can be dangerous).
Tests that are
not proven as valid include cytotoxic testing (in which foods are mixed
in a test tube with a sample of the person's blood and then the white
blood cells are observed for a change in shape) and "provocation
and neutralization" testing (in which the person is injected with a
food or other substance and, if there is an allergic reaction, they are
injected with more of the substance to "neutralize" the
response; this can be very dangerous!)
