Allergy Skin Testing
A skin test result, showing the typical wheals that show up with allergies.
This individual has lots of allergies!
This individual has lots of allergies!
Allergy Blood Test
Skin testing is generally considered the test of choice in the evaluation of allergies because it is a quick, accurate, nearly painless procedure. It
can easily be performed in the clinic setting and provides answers in less than 20 minutes. This test is minimally intrusive and is well tolerated by
young children, as well as adults!
To perform a skin test, drops of allergen extracts are placed on the forearms or the back and subsequently pricked with a plastic device or a
small needle. This feels like someone is pressing the tip of a pen or pencil onto the skin. There is no bleeding. Allergies reveal themselves as
red, itchy, mosquito like bumps called wheals. These itchy spots typically go away on their own in less than an hour.
Antihistamines (to include Allegra, Claritin, Clarinex, Zyrtec, Benadryl, and Atarax) will inhibit the formation of wheals during skin testing. It is
thus important to stop all antihistamines 5-7 days prior to the day when the skin test will be done. Steroid nose sprays (including Flonase,
Nasonex, Rhinocort, Nasocort) and asthma medications (any asthma inhalers, and the pill Singulair) have no affect on skin testing and should not
be stopped prior to skin testing. Click here for a full list of medications to stop before an allergy skin test.
can easily be performed in the clinic setting and provides answers in less than 20 minutes. This test is minimally intrusive and is well tolerated by
young children, as well as adults!
To perform a skin test, drops of allergen extracts are placed on the forearms or the back and subsequently pricked with a plastic device or a
small needle. This feels like someone is pressing the tip of a pen or pencil onto the skin. There is no bleeding. Allergies reveal themselves as
red, itchy, mosquito like bumps called wheals. These itchy spots typically go away on their own in less than an hour.
Antihistamines (to include Allegra, Claritin, Clarinex, Zyrtec, Benadryl, and Atarax) will inhibit the formation of wheals during skin testing. It is
thus important to stop all antihistamines 5-7 days prior to the day when the skin test will be done. Steroid nose sprays (including Flonase,
Nasonex, Rhinocort, Nasocort) and asthma medications (any asthma inhalers, and the pill Singulair) have no affect on skin testing and should not
be stopped prior to skin testing. Click here for a full list of medications to stop before an allergy skin test.
One way to evaluate for allergies is with a blood test called a RAST (RadioAllergoSorbent Test). This test works by measuring a sample of
blood in a lab for allergy antibodies to various allergens. Results are usually given as a number from 0 to 100. The higher the number, the more
likely that an allergy is present. The severity of a reaction, however, does not correlate with the numbers. That is: someone with a very high
number will not necessarily have a worse reaction than someone with a lower number. There are several different laboratory companies that have
developed their own RAST system (ImmunoCap, Immulite, Turbo RAST,...).
Is This Test Any Good?
Overall, the RAST is a pretty good test, but it is generally not considered as good as a skin test because it may not be as good in picking up
allergies. For example, with food allergies, a recent study tried to assess how good three different types of RAST were by looking at the results
of 60 individuals with known peanut allergies. The ImmunoCap and Immulite both missed 10 (16.6% of patients) and the Turbo RAST missed 9
( 15% of peanut allergic patients). In the same study, all three RAST systems also missed 15% of soy allergic individuals tested. Similar to the
skin test, up to 20% of results can be falsely elevated. It is thus important to interpret the results carefully.
When should I have a RAST done?
Typically, the allergy blood test is done when it is not possible to do a skin test. For example, the RAST might be done in an individual with
severe eczema, where the rash covers up the arms and the back, and there is not enough room to perform an adequate skin test.
Source
Wood et al. Accuracy of IgE antibody laboratory results Ann Allergy Asthma Immunol. 2007;99:34-41.
blood in a lab for allergy antibodies to various allergens. Results are usually given as a number from 0 to 100. The higher the number, the more
likely that an allergy is present. The severity of a reaction, however, does not correlate with the numbers. That is: someone with a very high
number will not necessarily have a worse reaction than someone with a lower number. There are several different laboratory companies that have
developed their own RAST system (ImmunoCap, Immulite, Turbo RAST,...).
Is This Test Any Good?
Overall, the RAST is a pretty good test, but it is generally not considered as good as a skin test because it may not be as good in picking up
allergies. For example, with food allergies, a recent study tried to assess how good three different types of RAST were by looking at the results
of 60 individuals with known peanut allergies. The ImmunoCap and Immulite both missed 10 (16.6% of patients) and the Turbo RAST missed 9
( 15% of peanut allergic patients). In the same study, all three RAST systems also missed 15% of soy allergic individuals tested. Similar to the
skin test, up to 20% of results can be falsely elevated. It is thus important to interpret the results carefully.
When should I have a RAST done?
Typically, the allergy blood test is done when it is not possible to do a skin test. For example, the RAST might be done in an individual with
severe eczema, where the rash covers up the arms and the back, and there is not enough room to perform an adequate skin test.
Source
Wood et al. Accuracy of IgE antibody laboratory results Ann Allergy Asthma Immunol. 2007;99:34-41.
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Picture of a skin test being placed on the back of a small child.
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