Ron Hines DVM PhD
To see what normal blood and urine values are, go here
For an explanation of causes of most abnormal blood and urine tests, go here
To see how tests are grouped, go here
To see how laboratories suggest specific gravity results be interpreted, go here
The Specific Gravity Of Your Cat And Dog’s Urine = SpGr, USG
Determining the specific gravity of your pet’s urine is the way veterinarians judge how concentrated it is.
The higher the SpGr number, the more concentrated your dog or cat ‘s urine is – the lower the number, the more dilute it is. Pure water has a specific gravity of 1.000. If your pet’s kidneys are healthy, its urine specific gravity will be constantly changing – but its kidneys will do their best to keep its SpGr between 1.015 and 1.060.
The number will go up when your pet is dehydrated and go down after it drinks water. It will tend to be highest just after it gets up in the morning.
Some Reasons Why Your Cat And Dog’s Urine Specific Gravity Reading Might Be High:
A normal or high urine specific gravity usually indicates that your pet’s kidneys are working well. But it can also be high if your pet has developed diabetes that has allowed sugar to leak into its urine or when the protein content of the urine is abnormally high.
When your pet’s urine specimens repeatedly test high for SpGr, it is often a sign that your pet is not consuming enough fluids throughout the day. That can be a major side effect of maintaining pets (particularly cats) on dry pet food. You see, many cats do not drink sufficient water even when you keep a bowel of water always available to them. Those cats stay marginally dehydrated producing high SpGr urines. The problem is less common in dogs. Dehydrated pets are thought to be at greater risk of developing kidney and bladder stones because the mineral salts that form these stones fall out of solution (precipitate) to form crystals (most commonly oxalate or struvite crystals) much faster in concentrated (high specific gravity) urine than in dilute urine.
Chronic dehydration due to lifestyle can elevate your pet’s urine specific gravity. That situation is thought to be most common in cats consuming dry cat food. In the wild, cats consume rodents that are about 75% water – so their need for additional water is minimal. The outward signs of chronic dehydration in cats have not been studies. In humans, signs of dehydration are often nebulous (vague). (ref1 , ref2, ref3) More severe dehydration often accompanies any disease in which apathy, weakness or reduced mobility occurs.
Other causes of dehydration that will elevate your pet’s urine specific gravity are vomiting, diarrhea, heatstroke and fever.
Some Reasons Why Your Cat And Dog’s Urine Specific Gravity Reading Might Be Low:
Persistently low urine specific gravity in your dog or cat is more common, and potentially more worrisome than high specific gravity. In older pets, it is often the first sign (along with increased water consumption = polydipsia) that the pet’s kidneys are failing. Persistently low urine specific gravity is called isosthenuric urine (1.008-1.012). That urine is more or less the same concentration (osmolality) as the pet’s blood plasma and it means that the pet’s kidneys can neither concentrate nor dilute urine once it has initially formed. Those pets are usually azotemic (= high BUN and creatinine) as well. You can have a look at a chart that shows how veterinarians try to sort out the causes of dilute urine here.
A 2015 study found that healthy cats should have a urine specific gravity of 1.035 or greater. That expected number decreased slightly as the cat’s aged and it was slightly lower in female cats. Urine submitted to outside laboratories often registered slightly higher values than those run with a refractometer at your veterinarian’s animal hospital. In older (≥ 9yrs) cats, lower-than-normal urine specific gravity results should be followed by a blood BUN and creatinine assay to screen the cats for possible kidney disease. (ref) If an SDMA test provides any additional useful information remains to be seen. (ref)
Unspayed adult female dogs and cats with a uterine infection (pyometra) often exhibit increased thirst and abnormally dilute (low SpGr) urine. That is due to toxins released by bacteria in their infected womb.
A number of conditions can cause your pet’s blood calcium levels to be too high. You can read about them here. That higher-than-normal blood calcium level can interfere with the ability of your pet’s kidneys to concentrate urine (ie low SpGr).
Dogs and cats with failing livers (hepatic insufficiency) sometimes also produce low specific gravity urine. Those pets will probably also have abnormal levels of blood blood albumin , bile acids, cholesterol and liver enzyme tests (ALT, AST, AP, bilirubin). Veterinarians are uncertain as to why liver problems affect the SpGr of your dog or cat’s urine.
Overactive Adrenal glands (Cushing’s disease = hyperadrenocorticism) or corticosteroid medications often cause low specific gravity urine in dogs. That is because corticosteroids act on the brain to increase thirst. (ref) The effect is less pronounced in in cats. Urine becomes more dilute because these pets drink more.
Hyperthyroid cats also often drink more and produce more dilute urine. It is not clear why that occurs.
Low blood potassium can also be associated with dilute urine.
Under active adrenal glands (Addison’s disease = hypoadrenocorticism) can also cause your pet’s urine to be too dilute. An ACTH test is in order when your pet’s urine is too dilute (low specific gravity) but the pet’s blood creatinine test is normal.
Diabetes insipidus, a rare form of diabetes, will also cause pets to produce a urine with low specific gravity. This disease is usually due to a failure of your pet’s pituitary gland to produce the hormone ADH.
Complementary Tests That Come To Mind: