Blood Testing For Pets Under 6 years old: Pre-anesthetic blood tests help ensure your pet’s liver and kidneys are working well, and ensure they have the appropriate levels of red blood cells, blood proteins, and blood sugar. Each of these things are important while under anesthesia. It is uncommon or younger pets to have issues with these parameters, but pre-anesthetic blood testing is the only way to be certain. Changes on blood tests are more common in older pets, so pets over 6 years old are required to have pre-anesthetic blood tests.