Allergy tests are used to find the substances that are causing an allergic reaction. An allergic
is an immune system response to a substance known as a trigger.
Who is a candidate for the procedure?
The healthcare provider may perform or order allergy tests on
people with suspected allergies. Skin testing is usually not done on children
younger than age 3. It may be done on
older individuals to determine the triggers that are causing allergic reactions in the person.
Before the procedure, a
person should not take medicines that could block skin reactions. For example,
an antihistamine may block a skin reaction.
How is the procedure performed?
There are many methods of allergy testing.
Skin tests are used to identify specific problem allergens. The healthcare
provider will select skin tests based on a person's particular history. Test
solutions are made from extracts of various inhaled, ingested, or injected
substances. Test solutions available include extracts from tree, grass, and
weed pollens; dust mites; animal dander; insect venoms; foods; and penicillin
and penicillin derivatives.
For the prick skin test, a small amount of the substance thought to
cause the allergic reaction is placed on the skin. This substance is called an
allergen. The skin is then pricked or scratched. This allows the substance to
get under the skin's surface. If the person is allergic, the skin will usually
get red and swell within about 20 minutes.
Another skin test is called an intradermal test. A small amount of
the allergen is injected beneath the skin. This more sensitive test is often
used when the prick test has produced a negative or uncertain result in
reaction to suspected inhaled allergens.
An elimination diet test may be used to diagnose food allergies. For this
test, a person goes several weeks without eating any of the foods that may be
causing the problem. Foods are returned to the diet one at a time. If allergic
symptoms come back after eating a certain food, that food is probably causing
the problem. If a specific food is suspected, it can be given to the person. If
a reaction occurs, this food is the likely cause.
The best way to test for food
allergies is by using an oral food challenge. This test can be used
even with small
children. The suspected food is removed from the diet. After 4 to 5 days, the
food should be eaten on an empty stomach. This is the best time to watch for a
reaction. Parents can keep a food diary for their children. This can help
the foods causing the problems.
Blood tests measure antibodies to a particular allergen
in blood. One blood test for allergies is called a radioallergosorbent test, or
RAST. In a true allergic reaction, substances called IgE antibodies appear in
the bloodstream. The body makes IgE antibodies to attack bacteria and other
harmful substances. RAST measures allergen-specific IgE. Compared to skin
tests, RAST has the disadvantage of limited allergen selection and reduced
A newer version of a blood test is called the Immunocap. A recent
study indicated that the Immunocap test was significantly more accurate than
the older blood tests. Another study compared the accuracy of skin testing to
blood testing for cat allergy. This study reported that skin testing and blood
testing were about equal in accuracy.
Other blood tests, such as an eosinophil count, may be used to
support an allergy diagnosis. Provocative testing is the direct application of
an allergen to the eyes, nose, lungs, or gastrointestinal tract by oral
administration. It may be helpful in cases where a person has had a large
number of positive skin tests. Provocative testing is the only way to check for
allergies to food additives. This type of testing may produce a severe allergic
reaction and is used rarely.