My dog was over 6 years old when we learned she had liver issues. She's almost 12 and managed exclusively on a home cooked low protein diet. We had initially put her on the Hill's L/D canned and she did well on that for years but due to other problems (allergies, food intolerances) we had to go to home cooked. Her BATs were very elevated and they came down with diet to near normal readings. The Protein C test is used to confirm Microvascular dysplasia (internal liver shunting at the microscopic level). Here is more info on the protein C test
Animal Health Diagnostic Center