May I ask why the vet needs to do a blood test? Where I am (UK), the only reason vets do blood tests is if the cat is ill or the owner requests it. It is not done routinely.
No cat likes going to the vet.
NC - does she have a name? She will take a while to settle in. Everything is new and strange to her. Of course, there will be hissing - it's a normal and natural thing cats do until they are accepted by the other cats and vice-versa. As I said in my previous post, the less human interference, the better. We, with our best intentions of not letting the hissing/growling/swatting get out of hand have a tendency to make things much worse. The cat(s) pick up on our feelings, think "If the human is scared, something terrible's going to happen so I'm scared too".
I have been a multi cat household for years, anything up to 13 cats at any one time while I was fostering. The longest I've kept my cats separated from a new cat was 5 days and that was because our last rescue was unneutered and had an injured leg. We caught him on the Saturday evening and by the following Thursday he'd had vet treatment, his leg fixed, was neutered and microchipped. We took him home, sat the cat carrier in the middle of the room to let our other cats have a good sniff for about 30 minutes then opened the carrier door. He (Oscar) came out, got hissed at by Lucky and that was that. A few days later they were all curled up on my bed (or should that be their bed - I'm only allowed to share a small part of it). Oscar and Otto are really good friends and can often be seen grooming each other.
Just relax. The cats will be fine
