Sorry....just seeing this post. Hopefully, my bumping it will make it more visible to others and you will get more responses.
I would hate to say you don't need a test that your vet is suggesting. I probably would do it just so that I would know for sure; but I know other people who have opted not to do it and treat as if it is mvd. Usually shunts have higher BATs. It could be microvascular dysplasia. I think I would at least for now, speak with the vet about putting him on a prescription liver diet. One of my pups who had high BATs and a negative scintigraphy is on Royal Canin liver diet and doing very well. She has MVD.
Here is a link that might be helpful to you:
http://www.vet.utk.edu/clinical/sacs...2013-04-10.pdf