What Is Urine Specific Gravity?

Urine specific gravity is a term used to describe urine concentration. When your pet has a urinalysis that is performed by a veterinarian, urine specific gravity is one of the many things measured for both cats and dogs. This test is also given to humans as a diagnostic tool. An animal's urine specific gravity measurement is used in conjunction with other test results to evaluate a pet's health. The measurement will help the veterinarian evaluate your pet's renal function by assessing whether ingested water is being excreted or sustained in the body at appropriate levels.

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.

The hydration level of an animal 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. It's important to note that a pet's urine specific gravity will fluctuate throughout the day, which is completely normal. This measurement is especially important for cats because felines typically do not drink enough water, and cats eating more dry food than wet food may not ingest enough moisture.

Health problems can affect both hydration and urine concentration. Vets will use urine specific gravity values in part to make medical diagnoses, but other test results are generally factored into the process. Urine specific gravity is a valuable tool to evaluate a pet's kidney function.

How Vets Measure Urine Specific Gravity

Urine specific gravity is measured using a tool called a refractometer. This instrument uses light to measures the density of the urine. Urine can be collected at home if possible, or at the veterinarian's office using specific techniques that will not hurt your pet and may also be used to access uncontaminated urine. Then a drop of the 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 though a lens to read the result.

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.

Normal Urine Specific Gravity in Pets

There is no one "normal" urine specific gravity in pets. The measurement varies greatly even in healthy animals. In general, the normal range in pets is 1.001 to >1.045. Cats and dogs will have slightly different ranges. Multiple urine samples may need to be collected throughout the day in order to get a better idea of the animal's normal range. Other lab tests are greatly needed to assess a pet's health.

Hydration status greatly influences an animal's urine specific gravity, but other factors can affect the levels. Certain medications and disease processes will affect the urine specific gravity as well. In general, concentrated urine usually means that an animal is dehydrated while dilute urine suggests that the kidneys are not functioning normally.

Abnormal Urine Specific Gravity in 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.

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.

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

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

Vets use urine specific gravity as a guide combined with physical examinations and an animal'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).