SKIN TESTING

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.  

BLOOD TESTING

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.

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