ADJ Watson, Sydney, Australia, and HP Lefebvre, National Veterinary School of Toulouse, France

USG measurement is used frequently in veterinary practice to help evaluate renal function by assessing whether water is being excreted or conserved in an appropriate fashionappropriately,, according to need. It is measured most conveniently using a refractometer, which provides an approximate guide to urinary solute concentration that is satisfactory for routine clinical purposes.
 

The USG can also help verify the presence of polyuria (USG is inversely related to 24 hour urine volume), assist evaluation of urinary losses of protein, bilirubin and glucose, and aid assessment of the patient's state of hydration.

A wide range of USGs can be encountered in healthy animals – 1.001 to >1.075 for dogs and 1.001 to >1.085 for cats, but – although values encountered typically for normally hydrated individuals are often closer to 1.015 to 1.045 for dogs, and 1.035 to 1.060 for cats. It is important to note that any USG value could be considered 'normal' in a patient, depending on certain other factors, such as the patient's hydration status and whether or not azotaemia is present.
 

In healthy animals, urine concentration can change substantially over time, and 2 to 3 fold variations have been observed within 2 hours in some dogs. Cats typically produce more highly concentrated urine, but similar variation might be expected for normal cats.

Excretion of urine that is concentrated like this indicates that significant modification of glomerular filtrate (which has specific gravity of 1.008 to 1.012) has occurred by means of active resorptive processes occurring in the renal tubules. A substantial number of functioning nephrons is needed to produce urine of this concentration, so the simultaneous presence of azotaemia suggests there is likely to be a large prerenal component to the azotaemia. If the patient is not azotaemic, This finding is generally considered incompatible with a diagnosis of renal failure, although substantial renal kidney disease could still be present, with loss of up to 2/3 of normal nephron function may still be present.ª However, USG values in some cats with chronic kidney disease (CKD) and azotaemia can be renal failure may be as high as 1.040 or 1.045, so renal failurekidney disease should still be suspected in a cats if these values are accompanied by azotaemia and/or dehydration.
 

The production of very concentrated urine (USG >1.050) suggests reduced renal renal perfusion by blood, compatible with hypovolaemia, haemoconcentration or heart failure.

Excretion of urine more dilute than glomerular filtrate requires that the kidneys to perform metabolic work to produce hypotonic fluid in the distal tubules,; and this is incompatible with renal failurethis does not occur in patients that have lost enough nephron function to cause persistent azotaemia, as in CKD. However, significantsubstantial renal kidney disease can still be present in non-azotaemic patient thatanimals that can producee dilute urine, with loss of up to 2/3 of normal nephron function.ª
 

Other possible causes of USG <1.008 include primary polydipsia, central diabetes insipidus and conditions causing tubular insensitivity to vasopressin (ADH). Tubular sensitivity to ADH is often impaired in pyelonephritis, pyometra, glucocorticoid excess, hypercalcaemia, hypokalaemia, hyponatraemia, liver failure, or erythrocytosis.
 

Impaired solute resorption also results in dilute urine. Conditions interfering with solute resorption include administration of diuretic agents, glucocorticoid deficiency (Addison’s disease), normoglycemic glucosuria (Fanconi's syndrome, primary renal (primary) glucosuria) and hyperglycemic glucosuria (diabetes mellitus, administration of solutions containing glucose). Renal medullary washout, such as observed in chronic polydipsia, can interfere with the kidney’s concentrating ability.
 

Note that USG <1.008 can occur in normal healthy animals that are excreting surplus fluid to maintain homeostasis, as in primary polydipsia. However, further investigation is warranted if urine is consistently dilute with repeated sampling; a water deprivation test and/or ADH response test might be considered here.

Patients producing samples with USG within this range often have adequate renal renal function, but similar these values can be associated also with partial impairment of renal function due to renal kidney disease, or to some other factor inhibiting the ability to retain water, such as partial deficiency or inhibition of tubular responsiveness to ADH.
 

If dehydration is evident, moderately concentrated urine can be considered to be 'inappropriately dilute', warranting further investigation as indicated below.
 

If hydration is normal and there is no other evidence of renal kidney or other disease, it may be useful to reassess USG at intervals before undertaking additional studies. Any: only animals failing to produce concentrated urine (USG >1.030 for dog, >1.035 for cat) requires further investigation: possible options include testing urine concentrating ability in response to water deprivation and/or ADH administration, determining glomerular filtration rate (GFR), renal ultrasound examination, and renal biopsy.

Failure to produce more concentrated urine in the presence of dehydration can indicate renal insufficiencykidney disease. Alternatively, a partial deficiency in production, release or activity of ADH might be responsible (see section on Dilute urine: USG <1.008, above). Amongst possible contributing causes are: diuretic drugs, glucocorticoids, glucosuria, renal medullary washout, pyelonephritis, liver failure, and major electrolyte abnormalities (low K or Na, high Ca).
 

If azotaemia is also present, primary renal kidney disease and failure are is even more likely. Another possibility, although less likely, is that the animal has prerenal azotaemia together with some other factor hindering water retention.
 

Patients with inappropriately dilute urine should be investigated further for renal disease once other possible causes are excluded. This could involve testing urine concentrating ability in response to water deprivation and/or ADH administration, evaluating clearance of creatinine or another suitable markermeasuring GFR, and/or performing a renal imaging studyies and/or a renal biopsy.

Urine is inappropriately concentrated if USG exceeds 1.007 when the patient is over-hydrated. The combination suggests substantial renal failurekidney disease because adequate renal function should lead to excretion of excess water and more dilute urine.
 

This situation is unusual and mostly encountered when using intravenous fluids to induce diuresis in a patient with oliguric or anuric renal failureacute kidney injury or end-stage chronic kidney disease.CKD.

USG values in this range occur from time to time in samples from healthy dogs and cats, but renal failurekidney disease should be suspected if dehydration and/or azotaemia are present as well.
 

If isosthenuria persists in subsequent samplings, a concentrating defect should be suspected and investigated as indicated for 'inappropriately dilute urine'.
 

ª Note: it is commonly said that urine concentrating ability will become compromised once approximately two-thirds2/3 of total nephron function has been lost, and that azotaemia reflects loss of at least three-quarters3/4 of overall nephron function. While these two fractions are useful conceptually, they should not be regarded as literally correct true in all circumstances. What tThey do reflect, however, are two fundamentally important aspects of kidney disease and dysfunction: firstly, that a large proportion of renal function must be compromised before functional changes become evident clinically, and, secondly, that chronic progressive kidney diseases can be expected to cause loss of urine concentrating (and diluting!) capacity before inability to excrete metabolic wastes becomes evident.

• Vonderen IK van, Kooistra HS, Rijnberk A. Intra- and interindividual variation in urine osmolality and urine specific gravity in healthy pet dogs of various ages. Journal of Veterinary Internal Medicine 1997;11:30-35
• Watson ADJ. Urine specific gravity in practice. Australian Veterinary Journal 1998;76,392-398