A twelve hour fast is not even the proper protocol for a bile acids test. Samples should be paired around a meal.
Here is an article from Dr. Center, the developer of the bile acids test:
PSVA and MVD Research Summary
Are you close to a vet school? Most vets don't see enough liver disease to have much experience diagnosing it. You can waste a lot of time and money on tests not performed correctly or worthless tests like an ultrasound.
Here are a couple more good articles for you:
Hepatic Vascular Disorders - WSAVA 2006 Congress Portosystemic Shunts FAQ
Please post the BAT results when you get them back.