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Allergy Testing
| Definition |
| Any of several tests used to determine
the substance(s) to which a person is allergic. |
| Alternative names |
| patch tests (allergy); scratch tests
(allergy); skin tests - allergy; RAST test |
| How the test is performed |
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 (allergens) on the skin (usually
the forearm, upper arm, or the back), 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.
A similar method involves injection of a small amount of allergen under
the surface of the skin (intradermal) and watching for a reaction at the
site. Skin tests are most useful for respiratory allergies (in which the
allergen is inhaled), 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. In this test, suspected foods and placebos (inert substances)
are given in a disguised form. The person being tested and the health
care provider are both unaware of whether the particular substance being
tested in that session is a placebo or a food substance (a 3rd party
knows the identity of the substances and identifies them with some sort
of code). This test requires several sessions if more than one substance
is under investigation.
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).
Other tests include immunoglobulin measurements (see serum globulin
electrophoresis) and the blood cell differential and/or absolute
eosinophil count (increased eosinophils can indicate the presence of
allergy). Provocation ("challenge") testing includes exposure
to the suspected allergen (for example, in the diet or by inhaling the
suspected allergen) under controlled circumstances. This type of test
can provoke severe allergic reactions. |
| How to prepare 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. |
| Risks |
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 (and closely related medications) 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!). |
| How 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). Some
food intolerances, in which there is an inability to digest a food
because of lack of appropriate enzymes, mimic allergies. Some drugs,
such as aspirin, can cause allergy-like symptoms but without the
formation of antibodies or the release of histamine. |
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