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The improvement of an itching rash with the introduction of a GFD should
suggest the possibility of dermatitis herpetiformis.  This can be
diagnosed by special biopsies done by a dermatologist that suspects the
condition. See earlier postings on the subject.
RAST testing is a very controversial topic with little hard scientific
basis for its use in diagnosing food allergies( this is apersonal opinion)
Serology testing for celiac disease is much more precise.  I would regard
a careful history , exclusion and rechallenge( perferable blinded as a
better test of allergies.  However these are not always possible or
practical.
The whole area of food allergies is also affected by food intolerances
which include lactose intolerance, chemical effects of foods, chocolate,
coffee would be two examples.
I regard CD as simple as compared to food allergies.
Joe Murray
(not medical advice)