Rimadyl is a non-steroidal anti-inflammatroy medicine that can cause liver damage so they commonly look to see how the dog's liver function is at baseline. Many dogs do just fine on it and can use it for a long-ish period of time without problems. Some do have problems from it so knowing that the pup's liver function is OK at the outset would be important to know. Mine have taken it for short term periods, such as post-dental work or with a limp of uncertain origin, with no problems. If she needs longer term pain management, you could ask about what other meds might be effective with less risk of side effects. I can't think what else they might be doing bloodwork to look for at this point but maybe there's something else I just don't know about.