The first blood that gets drawn is after a 12 hour fast...In order the blood results to be accurate, the dog has to be fasted 12 hours for the initial drawing of blood...
Do all dogs with shunts have high bile acids?
Dog with shunts will almost always have high bile acids 2 hours after eating, and usually at least 95% of dogs will have high bile acids after a 12 hour fast. Samples are taken at both time periods ("fasting" or "preprandial", and "fed" or "postprandial") for several reasons. Some dogs normally release bile acids in the middle of the night and therefore naturally have a higher than normal fasting sample. Other dogs may have fat in their blood ("lipemia") after eating, which can interfere with the test. If only one blood sample can be obtained, it is best to take it 2 hours after eating.
Do all dogs with high bile acids have shunts?
Bile acids can be increased with any liver disease. Bile acids can also be mildly increased in normal dogs, particularly in some breeds (such as Maltese) where chemicals in their blood interfere with the test. Most dogs with liver shunts have fed bile acids over 100 (normal <15-20). If the bile acids are only mildly increased or the animal seems normal, many veterinarians will simply rerun the test in 3-4 weeks.
Those two Q&A's were taken from
http://www.vet.utk.edu/clinical/sacs/shunt/faq.shtml
Also, I wanted to add that there is a new way to test for bile acids...It's a urine test. I think it's supposed to be fairly accurate also.