how to get a cat to pee...

[Lance]

Ember may or may not have a UTI. She's been peeing in one of the  
sinks in our rooms for a few weeks, but will also pee in her litter  
box. When the vet tried to get a sample from her yesterday, there was  
nothing to get. He sent some non-absorbant plastic "litter" home with  
us. I put the "litter" in her box after removing the normal litter  
and left for work this morning. It's after 9 p.m., and I still have  
nothing to pour into a container for the vet to test. Any thoughts on  
how to make sure I get a sample? I know I could test with pH strips,  
but I want the vet to be able to test for whatever he's looking for.

Thanks,
Lance