High levels of protein in the urine is called proteinuria. This is often the first sign of early renal (kidney) disease and indicates damage to a microscopic part of the kidney called the glomeruli. Basically, there is something wrong with the function of the kidney, that is then allowing proteins that should not escape from the blood into the urine..to do so. Hence, why there are elevated levels of protein in the urine.
Other things that can cause high levels of protein in the urine include some type of damage to the urinary tract, that causes hemorrhage (bleeding) into the urine. This blood in the urine will cause high levels of protein in the urine. If you dog does not show any blood in the urine, then this is not the cause of the high protein.
In cases like your dogs, where the patient is showing persistent proteinuria, but is not showing any clinical symptoms of renal disease, treatment to try and control the leakage of protein is considered beneficial. The treatments usually involve drugs called ACE inhibitors (a type of blood pressure medication), which by their actions can lower the leakage of protein into the urine. Diet change to a restricted protein diet (like Hill's k/d or Purina NF) is also beneficial. These diets also tend to be higher in omega-3 fatty acids, which leads to lower levels of protein in the urine.
As far as diagnosing what exactly the inciting damage in the kidney is...this involves renal biopsy. What is usually done, is your dog would first have a test called a coagulation panel to make sure that her blood can clot normally. Then she would have an ultrasound (sonogram) of her abdomen to look at the structure of her kidneys. If the doctor doing the sonogram felt that the kidney architecture looked abnormal, then a biopsy of the kidneys would be taken. This biopsy should be able to tell what the underlying problem is, and what is causing it. Then, the most accurate treatment plan can be formulated.
I am attaching a client information handout that I use in my practice that discusses this problem in more detail. I hope that you find it useful.Click Here
I hope that this information is of help to you, and I wish you the best of luck with your dog. Please let me know if I can be of further assistance.