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Has the vet tried. Ideally, the best urine sample to truly represent what is going on in th bladder is by cystocentesis. That's a direct needle stick in to the bladder. It is the most common way to get urine from a dog and the most accurate (more sterile sample). The best is when a vet has a ultrasound to help visualize the bladder and get the sample, and more and more vets do have ultrasound. So, I would confirm why the vet can't get the sample for you.
I do agree, it can be difficult to get the urine from a female dog.