DBlain, yes that can be challenging! I use a soup ladle and just slide it right under my girl as soon as she starts going. It's a little tougher with the boys.
15 is just right on the edge, so you are smart to follow up, but it is not definitive in itself.
Evaluating in conjunction with urine specific gravity and the fact that your girl has had elevated creatinine in the past will enable your vet to have a more clear picture of what is going on with the kidneys.
Diana