Urine Specific Gravity in Dogs and Cats

Urine specific gravity describes how concentrated a dog's or cat's urine is. This measurement, along with other test results, helps veterinarians evaluate a pet's health, including kidney function. Here's what you should know about your cat or dog's urine specific gravity.

What Is Urine Specific Gravity?

Urine specific gravity, often abbreviated as USG, is a measurement of urine concentration as compared to pure water. The specific gravity of pure or distilled water is 1.000. The specific gravity of urine is higher because of the substances found in urine. How much higher will depend on several factors.

When your veterinarian runs a urinalysis on your dog or cat, urine specific gravity is one of the many things measured. This test is also used in humans as a diagnostic tool.

Why Is Urine Specific Gravity Important?

A pet's urine specific gravity helps veterinarians assess a pet's kidney function by showing whether ingested water is being excreted or sustained in the body at appropriate levels. A pet's urine specific gravity will fluctuate throughout the day, which is completely normal.

Hydration is important to consider when evaluating the significance of a pet's urine specific gravity. A dehydrated animal typically has a high urine specific gravity while an overhydrated animal usually has a low urine specific gravity.

A urine specific gravity measurement provides important information to vets, especially for cats. Many felines do not drink enough water and stay on the brink of dehydration. Cats who eat more dry food than wet food may not consume enough fluids.

Health problems can affect both hydration and urine concentration. Veterinarians use urine specific gravity values in part to make medical diagnoses, but other test results are generally factored into the process.

What is a Normal Urine Specific Gravity for Pets?

There is no one "normal" urine specific gravity in pets. The measurement varies greatly even in healthy animals. In general, the normal USG range is 1.001 to >1.075 for dogs and 1.001 to >1.085 for cats. Multiple urine samples may need to be collected throughout the day to get a better idea of the pet's normal range.

What is an Abnormal Urine Specific Gravity for Pets?

Urine is considered concentrated if the USG is over 1.030 in a dog or over 1.035 in a cat. If the pet is already over-hydrated, then a USG over 1.007 is considered high.

Urine is considered dilute if the USG is less than 1.008. If the animal is dehydrated, then the urine is considered too dilute if it is less than 1.030 in dogs or more than 1.035 in cats.

Hydration status greatly influences an animal's urine specific gravity, but other factors can affect the levels, including certain medications and disease processes.

In general, concentrated urine usually means that an animal is dehydrated while dilute urine suggests that the kidneys are not functioning normally. If the USG is abnormal, your vet will need to run more diagnostic tests to determine why.

Isosthenuria

Urine specific gravity values in the 1.008 to 1.010 range are termed isosthenuria. This happens when the kidneys are unable to concentrate the urine more than that of protein-free plasma. A single reading in this range does not necessarily indicate kidney disease. The veterinarian will want to examine your pet, do blood work to check blood urea nitrogen and creatinine to assess kidney function, and consider other factors such as water intake, any medications, and concurrent diseases that may be present.

In the absence of other reasons for an isosthenuria urine, vets will likely want to repeat the urine specific gravity reading on the first urine sample of the morning. This is when urine is typically the most concentrated.

What Causes Abnormal Urine Specific Gravity in Dogs and Cats?

Abnormal urine specific gravity may indicate several medical conditions, including:

• Urinary problems
• Kidney disease
• Diabetes
• Dehydration
• Adrenal gland disease
• Thyroid disease in cats

Veterinarians use urine specific gravity as a guide combined with physical examinations and an pet's history to determine if more testing is needed. Those additional tests could include blood testing, a full urinalysis, specialized urine testing, ultrasound, and radiographs (X-rays).

How Vets Measure Urine Specific Gravity

Urine specific gravity is measured using a tool called a refractometer. This instrument uses light to measure the density of the urine. A drop of urine is placed on the glass of the refractometer and the cover is closed. The refractometer is held up to the light where it is refracted (changes direction due to the light). The lab technician looks through a lens to read the result.

Urine can be collected at home if possible, or at the veterinarian's office using specific techniques that will not hurt your pet. A vet may prefer to collect the sample to access uncontaminated urine.

Urine specific gravity may also be measured on a urine test strip, but the results are considered unreliable and are often contradicted when checked on a refractometer.