I had to get a urine sample from my elderly cat several years ago, and the vet told me just to place a saucer under him when he's peeing - to say that he was uncooperative is putting it nicely! He exited the litter tray in a major strop, which lasted for several days, and did not use his litter tray again for a week at least! I rang the vet and told them i couldn't get a sample and they said never mind we'll probbly take a blood sample next time he's in. He died, a fairly healthy old man, several years later, at sixteen and a half.