Hi Chris,
The only way to diagnose adverse food reaction is via an elimination diet. This mean taking a diet history and then picking sources that your dog hasn’t been exposed to. It is very difficult to do an elimination diet correctly. You must ensure the dog doesn’t get any other food/flavoring sources besides the test diet for 12 weeks. No flavored medications, toys, chews, cat box “snacks”, scavenged food outside Nada! And you must clear any existing skin infections.
Limited ingredient foods purchased at pet stores have been found to contain proteins not listed on the bag. They are not suitable for use for doing this important diagnostic test!! Once diagnosed you may choose to ” challenge” the dog to see what the dog reacts to. It then may be possible to find a OTC diet to trial.
I would only use home cooked, vet diets made for this purpose, or possible Rayne nutrition for an elimination diet.
If the dog is reactive to the base protein in the hydrolyzed diet there is a chance he/she will react to the hydrolyzed version. Therefore if a dog has been exposed to chicken before then hydrolyzed chicken diet may not be the ideal choice for a trial.
If your dog has environmental allergies in addition to adverse food reaction it will be harder to tell if there is a positive response to food trial if done when concurrently exposed to the environmental allergens.
Good Luck
