What are allergy tests?
Allergy tests are tests your health care provider can do to
find out what causes your allergy symptoms. Examples of
common allergy symptoms are sneezing, watery eyes, runny
nose, itching, swelling, hives, stomach cramps, and
diarrhea.
Why are these tests done?
Substances that trigger allergy symptoms are called
allergens. Allergy tests can help you know what substances
you are allergic to. Knowing which allergens cause your
symptoms can help you avoid these substances and reduce your
symptoms.
Consider having allergy tests if you have allergy symptoms
that you are not able to control with medicine. Your
provider may recommend that you start having allergy shots.
A mixture that contains the allergens identified in the
tests can be prepared for the shots. Your allergies can
then be treated by injecting the mixture into your skin in
tiny but increasing amounts over the course of many months.
Over time, the shots make you less sensitive to the
allergens.
How do I prepare for these tests?
- You may need to avoid taking certain medicines before the
tests because they might affect the test result. For
example, you may need to stop taking any antihistamines 3
to 7 days before the tests.
- Talk to your health care provider if you have any
questions.
How are the tests done?
The 4 main kinds of tests used to identify allergy triggers
are:
- skin tests
- blood tests
- food challenges
- elimination diets.
Skin tests: A test called the skin prick test is the most
common type of skin test. For this test, a drop of allergen
extract is put on the skin and then the skin is pricked with
a small needle through the drop of the extract. The test
can also be done with a pricking device that has been
presoaked in the allergen extract. Only the top layer of
skin is pricked. The test is usually done on the back or
the arm. The skin test is ready to check in about 15
minutes. If you are allergic to the allergen in any of the
extracts, a red bump that looks like a mosquito bite will
appear at the spot where the extract was placed. If the
prick test is negative or the test results are not clear, a
similar but more sensitive test called an intradermal test
may be done. For the intradermal test, a very small amount
of allergen is injected under the skin.
Blood test (RAST test): Blood tests are not done as often as
skin prick tests, but they can be useful in some cases. A
sample of your blood is sent to a lab for testing. The test
measures the amount of IgE antibody in the blood. The body
makes this type of antibody when trying to fight off
allergy-causing substances. The test results show whether
you are making antibodies to certain allergens and thus
whether you are allergic to those allergens.
Food challenges: To check for food allergies, your health
care provider may want you to do a food challenge test. For
this test, you are given gradually increasing amounts of a
food while your provider watches for symptoms. This test
should be done only by a trained professional who is ready
to treat you if you have a serious reaction to the food. In
cases of allergies that are not caused by IgE antibodies
(such as some gastrointestinal allergies), a food challenge
test may be the only good way to diagnose a food allergy.
Elimination diet: For another check of possible food
allergies, your health care provider may want you to avoid
eating certain foods for a few weeks to see if allergy
symptoms go away. During this time, you will need to keep a
record of the foods that you eat and any symptoms you have.
The diet is followed until all allergic symptoms are gone.
Foods are then added back to the diet one at a time. If
symptoms come back, you know which foods are safe to eat and
which foods to avoid.
What do the test results mean?
If the skin or blood test is negative for an allergen, then
you probably do not have an allergy to that substance.
If the skin test is positive for an allergen, it may mean
you are allergic to that food. However, sometimes a test
can be positive even if you are not allergic to the food.
The positive test result can be wrong sometimes because:
- You can sometimes continue to have a positive test result
for many years to an allergy you have outgrown.
- You are allergic to a different substance that has some
components similar to the allergen you were tested for.
For example, you might have a positive test for soy if
you have peanut allergy, or a positive test to wheat if
you have a grass pollen allergy.
Test results are only one part of a larger picture that
takes into account your medical history and current health.
Sometimes a test needs to be repeated to check the first
result. Talk to your health care provider about your
results and ask questions.
What happens after the tests?
An allergy specialist will interpret the results of the
tests and suggest ways your allergy might be treated.
Allergy testing is the most precise way to find what causes
allergic symptoms. Test results are only one part of a
larger picture that takes into account your medical history
and current health. Sometimes a test needs to be repeated
to check the first result. Be sure to discuss your results
with your health care provider and make sure you understand
how best to care for your allergy symptoms.
For more information contact:
This content is reviewed periodically and is subject to
change as new health information becomes available. The
information is intended to inform and educate and is not a
replacement for medical evaluation, advice, diagnosis or
treatment by a healthcare professional.
Copyright © 2005 McKesson Corporation and/or one of its subsidiaries. All Rights Reserved.
This
information is approved and/or reviewed by U-M Health System providers
but it is not a tool for self-diagnosis or a substitute for medical
treatment. You should speak to your physician or make
an appointment to be seen if you have questions or concerns about
this information or your medical condition.