Urine Specific Gravity in Cats (USG)
Urine specific gravity (USG) measures how well a cat's kidneys concentrate urine. Learn normal ranges, what isosthenuria means, and its role in diagnosing CKD.
Quick definition: Urine specific gravity (USG) measures how concentrated a cat's urine is, which reflects how well the kidneys can conserve water. A normal feline USG is roughly 1.035 to 1.060 or higher. A reading in the isosthenuric range of about 1.008 to 1.012, especially with elevated blood waste, is a key sign that the kidneys have lost their ability to concentrate urine.
Urine specific gravity is one of the most informative tests in a senior cat's workup, and it is easy to overlook because it lives on the urine report rather than the bloodwork. USG tells your vet whether the kidneys can still concentrate urine, which is something blood tests alone cannot show.
Cats evolved as desert animals and normally produce very concentrated urine. When that concentrating ability fades, it is often one of the earliest measurable signs that the kidneys are in trouble.
What USG Measures
USG reflects the amount of dissolved waste and minerals packed into the urine. Healthy kidneys pull water back into the body and excrete a concentrated, high-USG urine. As kidney tissue is lost, the kidneys lose this concentrating power and the urine becomes more dilute. A refractometer measures USG quickly on a fresh sample.
Normal Ranges and What They Mean
| USG Range | What It Suggests |
|---|---|
| 1.035 to 1.060+ | Well-concentrated, normal feline urine |
| 1.013 to 1.034 | Less concentrated; interpret with blood tests |
| 1.008 to 1.012 (isosthenuria) | Kidneys not concentrating or diluting; a CKD clue |
| Below 1.008 (hyposthenuria) | Actively dilute; consider other causes too |
These ranges are guides, not absolutes, and a single sample can be affected by recent drinking. Your vet reads USG alongside the broader panel in our senior cat normal lab values reference.
Why USG Is the Companion to Blood Tests
Blood markers like creatinine and BUN can rise from simple dehydration even when the kidneys are healthy. The difference shows up in the urine. A dehydrated cat with healthy kidneys makes concentrated urine, so the high blood waste is pre-renal azotemia. A cat whose blood waste is high while the urine stays dilute or isosthenuric has lost kidney concentrating ability, which points to true renal disease. This pairing is the heart of an early kidney diagnosis.
Understanding Isosthenuria
Isosthenuria, a USG around 1.008 to 1.012, means the urine is no more concentrated than the fluid that first filters out of the blood. The kidneys have essentially stopped adjusting it. In a senior cat with rising kidney values, persistent isosthenuria is a strong indicator of chronic kidney disease and helps your vet stage it using the IRIS CKD staging framework.
Other Reasons for Dilute Urine
Dilute urine is not always kidney disease. Hyperthyroidism, diabetes, excessive water intake, and some medications can also lower USG. If your cat is drinking a lot of water, USG helps your vet narrow the cause. The full kidney overview is in kidney disease in senior cats.
- An early-morning sample is usually the most concentrated and informative.
- USG is measured with a refractometer; dipstick readings are unreliable in cats.
- Non-absorbent litter can let you collect a home sample for your vet.
- Borderline results are often rechecked rather than acted on immediately.
This page is educational and complements, but does not replace, your veterinarian. Always discuss your cat's specific results with a professional who knows their history.
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Frequently Asked Questions
What is a normal urine specific gravity for a cat?
Healthy cats produce highly concentrated urine, typically with a specific gravity of 1.035 to 1.060 or higher. Cats are desert-adapted animals, so well-functioning kidneys concentrate urine efficiently. A USG below about 1.035, especially when paired with high blood waste, suggests the kidneys are losing their ability to concentrate, which is an important early clue to chronic kidney disease.
What does isosthenuria mean in cats?
Isosthenuria is urine with a specific gravity in the range of about 1.008 to 1.012, the same concentration as the blood filtrate. It means the kidneys are neither concentrating nor diluting the urine. In a senior cat, persistent isosthenuria alongside elevated BUN and creatinine is a classic sign of chronic kidney disease, because the kidneys have lost their concentrating ability.
Why is urine specific gravity important for diagnosing kidney disease?
USG shows whether the kidneys can still do their concentrating job, which blood tests alone cannot reveal. A cat can have rising BUN and creatinine from dehydration while the kidneys work fine, in which case the urine is concentrated. But high blood waste with dilute or isosthenuric urine points to true kidney disease. That is why vets nearly always pair a urine test with senior bloodwork.
Can a cat have a low USG without kidney disease?
Yes. Several conditions cause dilute urine besides kidney disease, including hyperthyroidism, diabetes, drinking large amounts of water, and certain medications such as steroids or diuretics. This is why your vet interprets USG in context, with blood tests, symptoms, and history. A single dilute sample is a reason to investigate further, not an automatic kidney diagnosis on its own.
Does the timing of the urine sample affect specific gravity?
It can. A sample taken first thing in the morning, after the cat has gone overnight without drinking, tends to be the most concentrated and the most informative. A sample collected right after the cat drank a lot of water may read more dilute. For the clearest picture, vets often prefer an early sample and may repeat the test if the result is borderline.
How is urine specific gravity measured?
It is measured with a refractometer, a small optical instrument that reads how much dissolved material is in the urine. The test is quick, inexpensive, and done on a fresh sample. Dipstick pads for specific gravity are unreliable in cats, so refractometry is the standard. The sample can be collected at the clinic or, in some cases, at home using non-absorbent litter.
My cat has dilute urine. What happens next?
Your vet will look at the whole picture: blood waste levels, SDMA, urine protein, thyroid status, and blood sugar, plus any symptoms. If dilute urine pairs with elevated kidney values, chronic kidney disease is likely and your vet will stage it using IRIS criteria. If blood waste is normal, they will look at other causes such as hyperthyroidism or diabetes before concluding anything.
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