I agree with Crystal. This is what Dr. Center says about the reliability of ultrasounds:
Abdominal ultrasound with color
flow doppler is one of the least
invasive imaging techniques, but
also one of the most difficult to
perform and interpret. Ultrasound
requires an experienced ultrasonographer
as shunts can be easily
missed, especially if the dog is small
or uncooperative. http://www.yorkiefoundation.org/purina1.pdf Ultrasonography can detect "classic" features: small liver, hypovascular intrahepatic portal system, large kidneys, uroliths, and in 60 to 90% of cases (depending on ultrasonographer skill) identification of an aberrant shunting vessel. Doppler color flow interrogation identifies PSVA associated turbulence in the vena cava. Otherwise, a slow, tedious, systematic search of the portal vasculature is required. Hepatic Vascular Disorders - WSAVA 2006 Congress
A Protein C test would definitely be the next step:
Comparative Coagulation -- NYS Animal Health Diagnostic Center