I am currently dealing with this with my 2-and-a-half-year-old. (“Currently”…as in just got the blood work back yesterday.) So, I feel your frustration.
Apparently, allergies can develop at any age, so, unfortunately, you can’t rule out potential allergens because the reaction is new. And, apparently, the blood test can’t be trusted to detect food allergies, so an elimination diet may be in your future. (I found this part out AFTER agreeing to the test. A scientific journal cited the statistical odds of a blood test correctly identifying a canine food allergy as “considerably worse than a coin flip.”)
Depending on which lab is running the blood test (HESKA in Colorado is supposedly the most accurate), the blood work SHOULD give you a good idea of whether there are environmental allergens in play. Luckily, my guy is not allergic to me (yup—an actual possibility—dogs CAN be allergic to humans). He IS mildly allergic to some other things I wouldn’t necessarily have predicted, such as cats (who knew???), flea saliva, and household dust. But, all in all, the test was largely inconclusive. So we’re back to square one and have more detective work in front of us.
In the meantime, we’ll continue bathing him weekly with an all-natural sulfate-free shampoo and oatmeal-based conditioner. We’ll also continue to wash ALL of his bedding and stuffed toys at the same time we bathe him (in HOT water), so there’s no cross-contamination if the culprit is truly environmental (e.g. dust, mites, fleas, etc.). This has helped somewhat: He’ll be blissfully itch-free for a day or two. I’ve also just found out that it may help to wipe your dog down with a damp towel after any outdoor walk, in case the problem is seasonal pollen. Oh—and we’re using a little olive oil in his food until we can rule out fish as the problem.
There are other, more invasive tests, and heavier-handed shampoos that your vet may prescribe if this persists. These include scratch tests and punch biopsies, and antibacterial shampoos if the constant scratching is opening up any wounds. And, of course, there’s the dreaded elimination diet.
Good luck! I’ll come back and post again if I find out anything enlightening.