Understanding Urine Specific Gravity in Pets

Adapted from the IRIS education article "Using urine specific gravity" by ADJ Watson (Sydney, Australia), HP Lefebvre (Toulouse, France) and J Elliott (London, UK), revised by J Elliott in 2022.

Why Measure Urine Specific Gravity?

Urine specific gravity (USG) is measured frequently in veterinary practice to help evaluate kidney function, by assessing whether water is being excreted or conserved appropriately according to the body's needs. It is measured with a refractometer and gives an approximate guide to the concentration of solutes in the urine that is accurate enough for clinical purposes.

USG can also help confirm polyuria (USG is inversely related to 24-hour urine volume), help interpret urinary losses of protein, bilirubin, and glucose, and help assess the patient's hydration status.

What USG Values Are Considered Normal?

A wide range of USG values can be found in healthy animals: 1.001 to more than 1.075 in dogs, and 1.001 to more than 1.085 in cats. Values in normally hydrated animals are typically closer to 1.015 to 1.045 in dogs and 1.035 to 1.060 in cats. Any USG value could be considered normal in a particular patient, depending on other factors, including the patient's hydration status.

In healthy animals, urine concentration can change substantially over time, and two- to three-fold variations have been observed within two hours in some dogs. Cats typically produce more concentrated urine than dogs, but similar variation might be expected in normal cats.

Concentrated Urine: USG Above 1.030 (Dog) or Above 1.035 (Cat)

Producing urine this concentrated shows that the glomerular filtrate, which has a specific gravity of 1.008 to 1.012, has been substantially modified by active reabsorption in the renal tubules. A large number of functioning nephrons is needed to produce urine this concentrated, so if azotemia is present at the same time, there is likely to be a large pre-renal component to it. This can be confirmed by checking whether the azotemia resolves when the animal is rehydrated. If the patient is not azotemic, this finding is generally considered incompatible with substantial kidney disease, although kidney disease with loss of up to two-thirds of normal nephron function may still be present (see the note at the end of this article). However, some cats with chronic kidney disease (CKD) and azotemia have USG values as high as 1.040 or 1.045, so kidney disease may still be suspected in a cat with these values if azotemia is persistent.

Very concentrated urine (USG above 1.050) can be seen when reduced blood flow to the kidneys results from hypovolemia, hemoconcentration, or heart failure. It can also occur in healthy cats that eat mainly dry food and drink little water.

Dilute Urine: USG Below 1.008

To excrete urine more dilute than glomerular filtrate, the kidneys must do metabolic work to produce hypotonic fluid in the distal tubules, reabsorbing solutes in excess of water. This does not happen in patients that have lost enough nephron function to cause persistent azotemia, as in CKD. However, substantial kidney disease can still be present in animals producing dilute urine if at least one-third of normal nephron function remains.

Possible causes of USG below 1.008 include primary polydipsia, central diabetes insipidus, and conditions that make the tubules insensitive to vasopressin (antidiuretic hormone, or ADH). Tubular sensitivity to ADH is often impaired in pyelonephritis, pyometra, glucocorticoid excess, hypercalcemia, hypokalemia, hyponatremia, liver failure, and erythrocytosis.

USG below 1.008 can also occur in healthy animals that are excreting surplus water to maintain balance, as in primary polydipsia. However, further investigation is warranted if the urine is consistently dilute on repeated samples. A water deprivation test or an ADH response test might then be considered to investigate the underlying disorder.

Moderately Concentrated Urine: USG 1.013 to 1.029 (Dog) or 1.013 to 1.034 (Cat)

Patients with USG in this range often have adequate kidney function, but these values can also be associated with partial loss of kidney function, or with another factor that limits the ability to retain water, such as a partial deficiency of ADH or reduced tubular responsiveness to it.

If the patient is dehydrated, moderately concentrated urine is considered inappropriately dilute and warrants further investigation, as described below.

If hydration is normal and there is no other evidence of kidney or other disease, it may be useful to recheck the USG at intervals before carrying out more tests. Only animals that fail to produce concentrated urine (USG above 1.030 in dogs, above 1.035 in cats) need further investigation. Options include testing urine concentrating ability in response to water deprivation or ADH, measuring glomerular filtration rate (GFR), kidney ultrasound, and kidney biopsy.

Algorithms developed by machine learning that predict the onset of azotemic CKD within 12 months in older, apparently healthy cats have used USG together with serum creatinine and urea (Biourge et al., 2020). A single USG measurement on its own cannot be used in this way (Jepson et al., 2009).

It has been suggested in the past that USG could identify cats that will develop azotemia after treatment for hyperthyroidism. Studies that evaluated this objectively have either not supported the idea (Williams et al., 2010) or have found USG to be less sensitive and specific than blood biomarkers such as creatinine and SDMA (Peterson et al., 2018).

Inappropriately Dilute Urine: USG Below 1.030 (Dog) or Below 1.035 (Cat) in a Dehydrated Animal

Failure to produce more concentrated urine when an animal is dehydrated can indicate kidney disease. Alternatively, a partial deficiency in the production, release, or activity of ADH may be responsible (see the section on dilute urine above). Possible contributing causes include diuretic drugs, glucocorticoids, glucose in the urine, renal medullary washout, pyelonephritis, liver failure, and major electrolyte abnormalities such as low potassium, low sodium, or high calcium.

If azotemia is also present, primary kidney disease and kidney failure are even more likely. A less likely alternative is that the animal has pre-renal azotemia together with another factor that prevents water retention, such as hypercalcemia.

Patients with inappropriately dilute urine should be investigated further for kidney disease once other possible causes have been excluded. This could involve testing urine concentrating ability in response to water deprivation or ADH, measuring GFR by the clearance of creatinine or another suitable marker, kidney imaging, or a kidney biopsy.

Inappropriately Concentrated Urine: USG Above 1.007 in an Over-Hydrated Patient

Urine is inappropriately concentrated if USG is above 1.007 when the patient is over-hydrated. This combination suggests substantial kidney disease, because adequately functioning kidneys would excrete the excess water as more dilute urine.

This situation is unusual. It is encountered mostly when intravenous fluids are used to induce diuresis in a patient with oliguric or anuric acute kidney injury or end-stage chronic kidney disease.

Isosthenuria: USG 1.008 to 1.012

USG values in this range occur from time to time in samples from healthy dogs and cats, but kidney disease and impaired kidney function should be suspected if dehydration or azotemia is also present.

If isosthenuria persists on later samples, a concentrating defect should be suspected and investigated as described for inappropriately dilute urine.

A Note on Nephron Loss

It is commonly said that urine concentrating ability becomes compromised once about two-thirds of total nephron function has been lost, and that azotemia reflects the loss of at least three-quarters. These fractions are useful concepts, but they should not be taken as literally true in all circumstances. What they do reflect are two fundamentally important aspects of kidney disease. First, a large proportion of kidney function must be lost before functional changes become clinically evident. Second, chronic progressive kidney disease can be expected to reduce the kidneys' ability to concentrate (and dilute) urine before their inability to excrete metabolic wastes becomes evident.

Further Reading

• Biourge V, Delmotte S, Feugier A, Bradley R, McAllister M, Elliott J. An artificial neural network-based model to predict chronic kidney disease in aged cats. Journal of Veterinary Internal Medicine 2020;34(5):1920–1931.
• Jepson RE (2017). Polyuria and polydipsia. In: Elliott J, Grauer GF, Westropp JL, eds. BSAVA Manual of Canine and Feline Nephrology and Urology, 3rd edition, pp. 8–23.
• Peterson ME, et al. (2018). Evaluation of serum symmetric dimethylarginine concentration as a marker for masked chronic kidney disease in cats with hyperthyroidism. Journal of Veterinary Internal Medicine 32(1):295–304.
• Wamsley H, Alleman R (2017). Complete urinalysis. In: Elliott J, Grauer GF, Westropp JL, eds. BSAVA Manual of Canine and Feline Nephrology and Urology, 3rd edition, pp. 60–83.
• Williams TL, Archer J (2016). Evaluation of urinary biomarkers for azotaemic chronic kidney disease in cats. Journal of Small Animal Practice 57(3):122–129.
• Williams TL, et al. (2010). Survival and the development of azotemia after treatment of hyperthyroid cats. Journal of Veterinary Internal Medicine 24(4):863–869.