The test results look indicative of mild MVD/HMD (Hepatic Multivascular Dysplasia)
From:
Diagnosis of Portosystemic Shunts
"Single congenital portosystemic shunts must be differentiated from multiple acquired shunts secondary to portal hypertension, and from hepatic microvascular dysplasia. Hepatic microvascular dysplasia (HMD) signifies a disorganization of the liver's microscopic architecture which is similar to that of dogs with single congenital shunts. HMD has been reported in small breed dogs such as the Yorkshire terrier, Cairn terrier, Maltese, cocker spaniel, and poodle. Dogs with HMD display biochemical, hematologic, and clinical changes consistent with portosystemic shunting but lack a macroscopic portosystemic shunt. Definitive diagnosis is by ruling out a macroscopic shunt through exploratory laparotomy, nuclear scan, or portography. Signs of HMD are managed by low protein diet; lactulose is added if necessary."
Generally, postprandial BAT Results < 100 tend to be indicative of MVD/HMD, and > 100 indicative of PSS (Portosystemic Shunt). Further testing would be by your decision, but many people who own dogs that have similar BAT results as your dog have chosen (with their vet's recommendation) to manage the MVD/HMD on diet alone, or diet and meds (if needed), with further testing for a more definitive diagnosis deemed not necessary, nor in the best interests of the dog.