Dog with allergies on food trial

Question
My dog was adopted from a shelter about 3 and half months ago and we
don't know anything about his history. He is a little over 1 year old, a
beagle/terrier mix. He was always very very itchy, but had no fleas.
Additionally, he began to lose his fur very fast, presented wide areas of
redness, had vomiting episodes, and at the height of it all was shaking
uncontrollably, which the vet thought it was related to the unbearable
itchiness.

On March 7th we started him on Hill's prescription z/d ultra-low allergen
food + Temaril-P 1 pill twice a day for 4 days, then 1/2 pill twice a day,
and now only 1/2 pill a day. He is doing much better. His hair is growing
back, he is not shedding at all, his skin is no longer red, he doesn't itch, etc.
Although the dog was not supposed to eat anything at all besides the
prescription food, my husband sneaked him pieces of roasted chicken breast
about a week or so ago. I had no idea until he confessed, half bragging about
how smart he was that he was able to proof beyond a reasonable doubt that
the dog is not allergic to chicken.

My question to you is if he is right. Can this be proof that he is not sensitive
to chicken even though it was only once (or twice, my husband is very soft on
the dog)? If he were allergic to chicken, would I see a return of the symptoms
right away or over a period of time if there was continuous exposure? Could
it be that he may be allergic to chicken but the Temaril-P suppressed the
symptoms and we have to wait until he's off the meds to know for sure what
kinds of foods he is allergic to?

My second question is whether or not you recommend allergy testing. My vet
has told me twice that it would be a waste of money, especially because he
thinks the dog has food allergies more than any other allergies. What is your
opinion as a dermatologist?

Answer
IF the dog is Z/d responsive, then adding one protein at a time to the diet will tell you if he is sensitive to that protein.  So by having your husband give chicken, you can probably say that the dog is not allergic to chicken. If you gave some steak and the dog started itching again, then you would know that it is the beef he is allergic to.

Allergy testing is a 50-50 proposition.  At his age, it would be rare to find lots of positive allergens.  But if there is one very high test result, you could target that allergen. The dog needs to be off of the Temaril-P for 30 days before testing. Do not do blood testing for food allergies because these results have no meaning.