Kidney Failure in Dogs

14 min read


Diagnosis of Kidney Failure


One of the first signs an animal will show when beginning to be affected by kidney failure is an increased thirst, which is known as polydipsia. Increased toxins and other metabolic waste products triggers sensors in the brain that the blood is too concentrated and through a series of chemical reactions the animal may have a sense of dehydration. Your dog, in turn, drinks more water to alleviate this sensation. Compounding this sense of dehydration is actual water loss through the kidneys above normal amounts due to the kidneys being inefficient in retaining water within the body. 


The increased thirst/water intake (polydipsia) also causes an increased urine output. Known as polyuria, the increased urine output seems unintuitive if the animal is actually affected with kidney failure.


Many pet owners have been baffled when the veterinarian mentions that the patient may have early kidney failure. They often respond "How can that be, its urinating a lot more than it usually does?"  What really is happening is that much more urine is being produced and eliminated however the urine is becoming more and more dilute; the urine is not bringing along all those toxins and waste products for removal from the body.


In order to make a diagnosis of renal failure your veterinarian will use two sources of data: a urine and blood sample. Checking one without the other may render an inccurate diagnosis. 


The Urine Sample


In nearly all cases of kidney failure the kidneys are unable to concentrate urine. That means the Urine Specific Gravity measurement (SpG) that indicates how concentrated the urine is compared to distilled water (SpG = 1.00) will display a dilute reading ... actually, very close to distilled water. 


Since the action of conserving water while allowing undesirable metabolites and toxins to remain in the urine is the job of the tubules in the kidneys, whenever the tubules are damaged water conservation is less efficient; therefore more water flows through the tubules unresorbed and washes away in the now dilute urine. 


Most cases of kidney failure display a SpG of about 1.008 to 1.012. Generally, a normal dog's urine SpG will be 1.020 to 1.040. 


If a water deprivation test is done, where the animal has no access to water for 18 hours, the urine specific gravity goes up (i.e., the urine becomes more concentrated).


Many cases of kidney failure also show protein or sugar in the urine where in most normal animals urine protein is scarce and no glucose is present. The loss, or lack of reabsorption of protein or sugar molecules back into the blood after an initial pass into the tubular fluid, places the animal in a negative protein/energy balance. This state shows up as weight loss and muscle wasting. And since these animals have a poor appetite, the added stress of protein and energy loss in the urine really tends to make the maintenance of normal body weight nearly impossible.


Bacteria and blood may show up in the urine samples of chronic renal failure patients. Infectious agents, red and white blood cells, epithelial cells from the lining of the kidney and bladder structures, crystals, and protein plugs called casts that arise from damaged tubules all may be commonly observed in urine samples. Conversely, some patients have such dilute urine and such thirst that a urine sample may have no detectable cells or debris but simply show a low Specific Gravity and very dilute urine.



(See normal ranges for dog blood chemistry values here.)


Two of the most useful chemicals that veterinarian measure to see if toxins are building up in the patient's body are Blood Urea Nitrogen (BUN) and Creatinine. Normal BUN levels in dogs seldom reach higher that 25 to 30 mg/dl. (Mg/dl means milligrams of material per 100 milliliters of blood.)  Many patients presented in renal failure have BUN levels of 90 or higher! Similarly, Creatinine, a  chemical normally present in the blood at levels less than 1.0 mg/dl, may rise to over 8 mg/dl.