How Do Vets Diagnose Atopy? Veterinarians diagnose atopy by looking at the dog’s symptoms and ruling out other causes of itching. To identify the specific allergens causing the reaction, vets might use two types of tests: intradermal skin testing and blood testing. In skin testing, tiny amounts of allergens are injected into the dog’s skin to see if they cause a reaction. In blood testing, a sample of the dog’s blood is analyzed to see if it contains high levels of antibodies that fight specific allergens.