When I had to collect urine from my previous cat, I had emptied out the litter pan (as suggested by my vet) and waited for her to use the litter box for that purpose. She wouldn't go in but was pacing back and forth so I knew she had to go. So I called the vet right away and they recommended that I place a very small amount of litter in the pan which would trigger the cat's instincts to go in it. It worked like a charm! As she positioned herself to pee, I was able to collect the urine on a plastic spoon and then poured it into a urine speciman cup. I would have preferred to follow what the above posters suggested, but my vet did not offer that option and I didn't even know you could do it at the vet (but I am sure it costs more money, yet prob much more accurate).