Urine osmolality in cats and dogs

What it shows

Specific gravity and osmolality both describe how concentrated urine is, but they measure different things. Specific gravity compares the density of urine with water, so heavy molecules such as protein push it up more than their number warrants. Osmolality depends only on the number of dissolved particles, which makes it the more accurate measure of how well the kidneys concentrate urine. It is measured with a special instrument, an osmometer, by freezing point depression: the more particles, the lower the temperature at which the sample freezes.

In everyday urine the two agree well: in a study of dog and cat urine, refractometer readings and osmolality were very closely correlated. They drifted apart when urine contained a lot of protein (above about 1 g/L in that study). A rough estimate from specific gravity is the last two digits multiplied by 36 (1.030 → about 1,080 mOsm/kg), but the same specific gravity can correspond to quite different osmolalities.

When it is measured. Osmolality is used when the vet needs to know exactly whether the kidneys can concentrate urine — for example during a water deprivation test. In this test water is reduced gradually under close daily supervision to see whether urine becomes concentrated. It is done only in animals that are not dehydrated and have no kidney disease, because the test itself carries a risk of dehydration. A response test with a synthetic form of antidiuretic hormone (ADH) may follow.

Higher than reference — what it may be a sign of

High urine osmolality is usually not a problem in itself: it means the kidneys can save water. It is expected in an animal that is short of water. In a dehydrated animal with raised creatinine or urea, well-concentrated urine makes a cause outside the kidneys more likely.

Lower than reference

• About the same as blood plasma (roughly 300–320 mOsm/kg) — the kidneys neither concentrate nor dilute urine (isosthenuria). A fixed value like this, especially in a dehydrated animal or one with raised kidney values, strongly suggests kidney failure.
• Below the level of plasma — the kidneys can dilute but not concentrate (hyposthenuria). This is typical of diabetes insipidus: central, when too little ADH is produced, and nephrogenic, when the kidneys do not respond to it.
• Other causes of dilute urine — drinking a lot, fluid therapy, diuretics and corticosteroid medicines; the vet takes such medicines into account when reading the result.

Look at together with

• Urine specific gravity — the everyday measure of the same property; osmolality refines it.
• Plasma osmolality and sodium — sodium is the main contributor to plasma osmolality; the vet compares urine with plasma.
• Creatinine and urea — whether the kidneys filter normally.
• Urine protein — a lot of protein makes specific gravity read higher than osmolality would suggest.
• Water intake and weight — during a water deprivation test, body weight, hydration and urine are checked every day.

Units and conversion

Urine osmolality is reported in milliosmoles per kilogram (mOsm/kg); some forms write mOsm/kg H₂O, which means the same. This is the only unit, so there is nothing to convert. It should not be confused with specific gravity — a number like 1.030 without a unit: both lines describe how concentrated urine is, in different ways. In the app the value keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form, and for osmolality it is wide, because a healthy animal’s urine changes with drinking and food. For orientation only: in one study, healthy dogs had urine osmolality of 689–2,020 mOsm/kg and healthy cats 760–1,860 mOsm/kg, while plasma is about 300 mOsm/kg. The vet usually reads urine osmolality against plasma osmolality and the animal’s hydration, not as a single number.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Urinalysis: concentrating ability
2. Merck Veterinary Manual — Diabetes insipidus in animals
3. Mösch M et al. Influence of preanalytic and analytic variables in canine and feline urine specific gravity measurement by refractometer. J Vet Diagn Invest, 2020 (PMC)
4. Whitbread TJ. Urinalysis. Merck Veterinary Manual
5. eClinPath (Cornell University) — Sodium
6. IDEXX — Complete urinalysis panel: interpretation guide