Allergy Skin Testing
A skin test result, showing the typical wheals that show up with 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 one hour. 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 little 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.
An other way to evaluate for allergies is with an allergy blood 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.
Is This Test Any Good?
Overall yes, the allergy blood test is felt to be pretty good, though it is generally not considered to be as good as a skin test because it may not be
as good in picking up allergies. For example, with food allergies, one study tried to assess how good three different types of blood tests 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. What this means is that no blood test is perfect, and they should
be interpreted by an experienced allergist to determine what the results really mean.
When should I have a blood test done?
Typically, the allergy blood test is done when it is not possible to do a skin test. For example, it 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.
Wood et al. Accuracy of IgE antibody laboratory results Ann Allergy Asthma Immunol. 2007;99:34-41.
Updated 18 June 2013
Picture of a skin test being placed on the back of a small child.
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