Easy for me to say, but please try not to worry. The test is not a big deal. The vet will do a blood draw, feed Rocco, wait 2 hours after feeding, do a second blood draw. The end. A 12-hour fast is no longer a "must do". Although it won't hurt most dogs to go 12 hours without food if I had a very small one prone to hypoglycemic episodes I would not fast that dog for 12 hours. Also, bring whatever food Rocco normally eats with you. Most vets will tell you that the dog must eat a special food, usually Hill's A/D, but Dr. Center (the guru of liver disease) recommends feeding dogs whatever food they normally eat. The key is to get them to eat at least a few tablespoons of food. Better to do that with something they are used to eating and better not to have to force feed, thus causing a stressful situation for the dog. In fact, if Rocco is a picky eater who refuses to eat for the vet, ask them to call you so that you can go feed him. Good luck and keep us posted.
And, by the way, I have never ever heard of more than one puppy in a litter that has liver shunt. Others may have MVD, but multiple shunt puppies in one litter is extremely rare in our breed.