Urine specific gravity (USG) in cats and dogs
Urine specific gravity (USG) shows how concentrated the urine is — in other words, how well the kidneys hold on to water. It is one of the first lines a vet reads on a urinalysis, because the other urine results are hard to interpret without it. A single value means little until it is read next to the animal’s hydration and blood kidney values.
What it shows
USG compares the density of urine with the density of pure water (1.000). The more substances are dissolved in urine, the higher the number. Healthy kidneys concentrate urine when the body needs water and dilute it when there is water to spare, so USG changes during the day with drinking, food and hydration.
Vets measure USG with a refractometer, a small optical instrument. According to the Merck Veterinary Manual, the specific gravity pad on reagent strips is not valid in animals, so a USG read from a dipstick is not reliable; cat urine is read on a separate cat scale of the refractometer. USG also frames the rest of the urinalysis: trace protein or a few cells mean more in dilute urine than in concentrated urine.
Higher than reference — what it may be a sign of
- Not enough water in the body: a dehydrated animal with working kidneys makes concentrated urine. eClinPath calls urine adequately concentrated above 1.030 in dogs and above 1.040 in cats when the animal is dehydrated or azotemic (raised creatinine or urea); in an azotemic animal such urine usually points to a cause outside the kidneys (prerenal azotemia).
- Glucose or protein in urine: dissolved glucose and protein add to the reading, so in diabetes USG can stay fairly high even though the animal passes a lot of urine (Merck).
- X-ray contrast: iodine-based contrast agents such as iohexol can falsely raise the refractometer reading (eClinPath).
Lower than reference
One dilute sample is not necessarily a problem — after a long drink urine is naturally diluted (VCA). It matters when it persists, or when the animal is dehydrated or has raised kidney values. eClinPath describes urine fixed at about 1.008–1.012 (isosthenuria — the same concentration as blood plasma) as often seen with kidney failure, and urine below 1.008 (hyposthenuria) as a sign that the kidneys can dilute but cannot concentrate. Causes of dilute urine listed by Merck and eClinPath include kidney disease, diabetes mellitus, hyper- and hypoadrenocorticism, high calcium, hyperthyroidism, liver insufficiency, low potassium, diabetes insipidus, and diuretic or corticosteroid medicines.
Look at together with
- Creatinine and urea — raised blood kidney values with inadequately concentrated urine fit a kidney cause, though eClinPath lists exceptions; with adequately concentrated urine a cause outside the kidneys is more likely.
- SDMA — a blood marker that rises earlier than creatinine.
- Urine protein and UPC — the same dipstick protein result means different things in dilute and concentrated urine.
- Urine glucose — glucose raises the reading and diabetes increases urine volume.
- Drinking and fluids — how much the animal drinks and whether it has recently had fluids.
Units and conversion
USG is a ratio of two densities and has no unit. It is written with three decimals — 1.005, 1.025, 1.045 — and forms may print it as USG, SG or “specific gravity”. The number reads the same in every lab and needs no conversion. Temperature changes the result slightly: density falls as urine warms (eClinPath), which is why clinics use temperature-compensated refractometers.
Reference range
The reference range is on your lab’s form. USG is always read in context: the same number can be fine in an animal that has just drunk and worrying in a dehydrated one. Persistently dilute urine is a reason for further tests (VCA). eClinPath notes that the first morning urine is often suggested in dogs as the most concentrated sample, and that in one study first-morning USG in the same healthy dog varied by up to 0.015 within two weeks — so two results a little apart are not necessarily a change.
Cats and dogs
Cats normally make more concentrated urine than dogs: eClinPath’s cut-off for adequate concentration in a dehydrated or azotemic cat is above 1.040, and refractometers have a separate cat scale. Cornell notes that low USG is one of the earliest signs of chronic kidney disease in cats, and that recently given fluids can make it falsely low. In cats with Cushing’s syndrome, which often comes with diabetes, Merck notes that USG usually stays above 1.020 because glucose raises the reading.
In dogs eClinPath’s cut-off for adequate concentration in a dehydrated or azotemic animal is above 1.030. Dogs with Cushing’s syndrome usually drink a lot and have dilute urine (VCA, Merck).
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University) — Urinalysis: concentrating ability
- Whitbread TJ. Urinalysis. Merck Veterinary Manual
- Cornell Feline Health Center — Chronic Kidney Disease
- VCA Animal Hospitals — Urinalysis
- Greco DS. Cushing Disease (Pituitary-Dependent Hyperadrenocorticism) in Animals. Merck Veterinary Manual
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.