Here's a big pet peeve that vets have. A dog is scheduled for an appointment for abnormal urinating. When the client brings the dog in they let them urinate in the parking area before bringing the pet inside for its appoitnment. Then when we try to collect a sample for analysis the dog is empty and can't provide one.
To me it seems like common sense that if your pet is having difficulties urinating or the urine looks abnormal that the vet will want to perform a urinalysis to see if there is an infection, bladder stones, or other problems. If your dog is allowed to urinate just before being seen, then there isn't any urine to analyze. Maybe that's just me and it's not so obvious to most people.
Many things can cause a change in frequency of urinating, urine apperance, and other abnormal bathroom habits. While we may have some suspicions based on an exam, there is no way to tell for certain without performing tests on the urine. We can identify infections, suspect diabetes, analyze part of kidney functions, and get a hint about bladder stones with a simple urinalysis. In fact, we're usually hesitant to prescribe antibiotics for frequent urinating because we've all see cases where it's due to something other than an infection.
And if we can't get urine, we can't run these tests. This may mean keeping the dog in the clinic for several hours to allow them to build up more urine, or even getting it in the next day. Sometimes the only option is to have the owner collect a fresh urine sample and drop it off. In any of these cases it can be frustrating to the owner because we can't make an immediate diagnosis and start treatment.
If you're a pet owner and your dog or cat is scheduled to see the vet for urinary problems, please do yourself and the doctor a favor. Do everything you can to keep your pet from urinating for at least several hours before the appointment.