This is interesting because when we were going to Childrens, they do their own screening, and testing, temperament, and physical. They not only require you to get your normal Vet visits, but they also require you to go to their Research Lab. and have their own Vet run tests every 3-6 months, at their expense. This keeps everyone honest, helps to not add extra expenses on volunteers, and safeguards the hospital and their patients. Dogs have to have a complete bath within 24 hours before they visit, and have to wear a jacket through the hospital, til they get to the area they are visiting. They even offer certain programs to volunteer dogs who acquire Cancer, etc. if it is available, because they do their own research.
It probably helps that the person in charge of the program is a Newfie Person, and worked very hard to get it implemented.
I guess the requirements, all depend on where you go, and what organization you go through.
Susan