Just as we do in human hospitals, requiring pre-surgical bloodwork is an attempt to find subclinical problems before they cause problems during surgery. In pets, a pre-anesthetic blood screen usually looks at blood glucose, and a few tests to evaluate liver and kidney function. Well-functioning liver and kidneys are needed to remove the by-products of anesthesia from the body. Another test, that only some veterinary offices perform before surgery, evaluates clotting times – if your pet cannot clot its blood during surgery, it may bleed to death. This is an exceptionally uncommon risk during surgery, unless your dog is a Doberman, or a few other breeds, or ate rat poison 2 weeks ago.
So, if you do not do bloodwork before surgery and there is a sub-clinical liver, kidney or clotting problem, you will not know, and there may be problems. These problems can range from having a hard time recovering from anesthesia to not waking up at all. However, just as they have discovered in human hospitals, bloodwork is not definitive for possible problems – the bloodwork can be completely normal but there can still be surgical complications. Conducting pre-anesthetic bloodwork helps reassure us surgery is more likely to be safe, but it’s not a free ticket.