Before food allergy can be diagnosed, other causes of itching and skin disease, such as flea bite hypersensitivity and inhalant allergies, will need to be ruled out as possible causes of the itching and gastrointestinal disorders.
The best tool for definitively diagnosing food allergy is a food elimination diet, selected to contain a single protein and carbohydrate source to which that the animal has not been previously exposed. The special diet must be fed exclusively for a minimum of 13 weeks. If they have a food allergy, most animals will begin to show signs of improvement within four weeks of the diet, but will require feeding of the elimination diet for a longer time in order to eliminate symptoms. Following the elimination diet, the original food is fed to the animal to confirm that it does incite an allergic reaction. If the animal is allergic to it, the return of clinical signs will occur within 10 days of restarting the original diet.
Once the food allergy has been established, the exact allergen is investigated by placing the animal on the elimination diet until it is free of allergic symptoms. Subsequently, single food ingredients are added to the elimination diet and the animal is assessed for an allergic reaction. This process is repeated for each ingredient. Commonly suspected allergens include beef, chicken, fish, pork, and lamb meats; eggs and dairy products; and carbohydrate sources such as corn, wheat, soybeans, rice, and potatoes.
