Urine Specific Gravity

Dilute urine — a low USG — means the kidneys are not concentrating well, or the body is deliberately shedding water. Context is everything:

• With normal blood markers and lots of drinking, it may reflect simple factors (recent fluids, diet, medicines) or point towards conditions that cause increased thirst and urination, which then need investigating.
• With raised creatinine or SDMA, inappropriately dilute urine is a red flag for kidney disease — because failing kidneys lose the ability to concentrate. Confirming that the urine is dilute when the blood says it should be concentrated is often what tips a picture from "watch" to "act".

A single dilute sample can be normal (a pet that has just drunk a lot); it is dilute urine that does not fit the blood picture, or that persists, that carries weight.

Highly concentrated urine — a high USG — usually reflects the kidneys doing their job well and conserving water, often because the pet is a little dehydrated. It is generally reassuring about concentrating ability. Very concentrated urine can occasionally accompany conditions that need attention, but on its own a good USG is a point in the kidneys' favour.

Loss of concentrating ability is one of the earliest functional signs of chronic kidney disease, often appearing before the pet is obviously unwell. As CKD advances, USG tends to drift towards the concentration of blood itself (isosthenuria) because the damaged kidneys can no longer move water effectively. Tracking USG alongside SDMA, creatinine and UPC gives a fuller, earlier read on kidney health than blood markers alone, and a falling ability to concentrate is a meaningful part of the trajectory.

Cats are desert-adapted and normally produce very concentrated urine, so a healthy cat's USG runs higher than a dog's. That makes a cat that has lost concentrating ability — producing dilute urine — a particularly important finding, and one of the early clues to the CKD that is so common in older cats. Dogs have a wider normal range, so the interpretation always factors in what is normal for the species.

USG is measured on a urine sample with a refractometer (not the dipstick pad, which is unreliable in pets). It has no units — it is a ratio of the urine's density to water. The most useful sample is one collected before fluids or a big drink; timing and hydration change the reading, so it is always interpreted alongside how the pet was sampled.

Because USG so often changes the meaning of a blood result, the single most useful thing you can do when kidney markers are questioned is to get a urine sample tested alongside the blood. If the urine is inappropriately dilute for the blood picture, discuss a fuller kidney work-up with your vet; if it is well concentrated, that is a reassuring counterweight to a borderline blood value.