Question: What is USG (Urine Specific Gravity) and what does it measure?

Answer:

USG is one of the parameters that are measured when a urinalysis is performed. USG stands for Urine Specific Gravity. In this test, it measures the concentration of the urine.

What Urine Specific Gravity Values Measure

The USG will vary throughout the day, and day to day, depending on the animal's hydration status and overall health.

Urine that is very concentrated usually means that the animal is dehydrated, very dilute urine may suggest that the kidneys are not functioning as they should, or it could be the result of a medication, another disease process (liver disease, cancer, pyometra), or drinking excess amounts of water.
There is a wide range of "normal" USG values for dogs and cats. Repeat readings, in addition to bloodwork (chemistry profile), are often necessary to gather the information necessary for a complete picture.

Measuring Urine Specific Gravity

Specific gravity of urine is measured using a refractometer. This instrument measures the density of urine, as compared to pure water. A drop of urine is placed on the glass, the cover closed, and the values are read by looking through a viewfinder. Here is a view through a refractometer.
Urine specific gravity may also be measured on a urine test strip, but the results are not dependable for this parameter of the urinalysis.

Urine Specific Gravity Values

Reference ranges vary for canine and feline USG values.

In general, here are some common readings in veterinary medicine:

• Dog: 1.016–1.060
• Cat: 1.020–1.040
• Horse: 1.025–1.060
• *From the Merck Veterinary Manual - Urine Volume and Specific Gravity table

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. Your veterinarian will want to examine the patient, do bloodwork (checking blood urea nitrogen and creatinine to assess kidney function), and consider other factors (water intake, any medications or concurrent diseases) that may be present.

In the absence of other reasons for an isosthenuric urine, your vet will likely want to repeat the USG reading on a "first morning" urine sample when urine is typically most concentrated.

Learn more:Kidney Disease in Dogs and Cats