This article is from the Canine Allergies FAQ, by Cheryl Minnier with numerous contributions by others.
If a dog has the above symptoms and responds well to the treatment
measures outlined below, no further diagnostic tests may be needed. If
the problem is severe and does not respond to simple measures, allergy
skin testing can be done. A portion of the skin is shaved and a
variety of substances are injected into the skin to see if they
provoke a reaction. If so, an individual series of injections are
formulated to give the dog over a period of time (there are blood
tests designed to identify allergens without the skin testing, however
their efficacy had not been proven. They should be reserved for cases
where skin testing is not possible).
 
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