According to the University of Tennessee study, Yorkies were found to be thirty-six times more likely to have a liver shunt than all other breeds combined. Size is not the issue: genetics are.
Dr. Sharon Center, the developer of the bile acids test, recommends all Yorkie puppies be BAT tested. This is from page 22 of her seminar handout:
"2: The best approach to avoid "over diagnosis" is to test bile acids in young dogs of highly affected breeds (at 4 mths of age) while they are clinically healthy and before they are adopted into pet homes. Highly affected breeds include: Yorkshire Terrier, Cairn Terrier, Maltese, Tibetan Spaniels as well as many other "terrier" type breeds (Miniature Schnauzer, Lhasa Apso, Shih Tzu, Dachshund, Bichon Frise, Pekingese, Toy and Miniature Poodles, and Havanese and others). Proactive assessment of serum bile acids will limit the awkward circumstance imposed when an MVD dog, with minor health issues, is suddenly recognized to have abnormal bile acids by a pet owner's veterinarian. This circumstance can lead to unnecessary diagnostic confusion and unwarranted invasive tests such as liver biopsy and portovenography. How old dogs should be at he time of initial testing has not been established. Typically, abnormal bile acids DO NOT normalize as a dog ages ..... " (all emphasis added by Dr. Center)
Dr. Center has recently changed her recommendation from 16 weeks to 18 weeks as her continuing research has indicated the extra two weeks give the organs a little longer to develop and the test is more accurate.
It sounds like this breeder does not test her breeding dogs for liver disease and is uninformed about shunts in Yorkies. I would pass on this breeder.
Here are a couple of links for you from two of the top experts, both of which explain how common liver shunts are in Yorkies.
Hepatic Vascular Disorders - WSAVA 2006 Congress Portosystemic Shunts FAQ