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Urine specific gravity measurement and interpretation in veterinary medicine
A look at the indications for this important part of a urinalysis and how to interpret the results. (Part two of a four-part series)



The root word sthen is Latin for the English word strength. Hypersthenuria, hyposthenuria and isosthenuria are terms that depict the solute concentration (or strength) in urine compared with the concentration of solute in glomerular filtrate:

  • Hypersthenuria (also called baruria) depicts urine of high specific gravity and osmolality compared with glomerular filtrate.
  • Hyposthenuria depicts formation of dilute urine with a specific gravity and osmolality that are significantly lower than those of plasma and glomerular filtrate.
  • Isosthenuria depicts urine with a specific gravity and osmolality similar to those of (or with the same strength as) plasma and glomerular filtrate.

Complete loss of the ability to concentrate or dilute glomerular filtrate according to body need is sometimes referred to as fixed specific gravity.

In the context of quantifying urine concentration, terms that are more useful than hypersthenuria are 1) maximum urine concentration, 2) functionally adequate urine concentration and 3) inappropriate urine concentration (Table 1). There is significant species variability in maximum and functionally adequate urine concentrating capacities.

Osmotic activity in extracellular fluid and urine

Sodium, chloride and bicarbonate account for about 90 percent of the osmotic activity of extracellular fluid. Nonelectrolytes such as urea, proteins and glucose account for the remainder of the osmotic activity (10 percent). Sodium, chloride and urea account for most of the osmotic activity in urine.

There is usually no significant difference between the osmolality (specific gravity) of uncentrifuged urine and of centrifuged urine because cells, casts and os on do not contribute significantly to osmotic pressure. There is no significant difference between serum and plasma osmolality since fibrinogen does not exert a significant osmotic effect. However, the quantity and type of anticoagulant used to obtain plasma may be of significance. For example, EDTA may contribute 5 to 20 mOsm/kg to plasma osmolality, depending on the amount of blood in a 2-ml Vacutainer (BD) tube. Heparin is usually used as the anticoagulant for plasma osmolality determinations.

Normally the osmotic concentration of urine is variable, depending on the fluid and electrolyte balance of the body and the nitrogen content of the diet. Species differences in the ability to concentrate urine are also significant (Table 1). Interpretation of urine osmolality is usually enhanced when the values of serum or plasma osmolality are also available. The osmotic concentration of plasma, serum, interstitial fluid and transcellular fluid is about 280 to 310 mOsm/Kg of water. The osmotic concentration of glomerular filtrate is about 300 mOsm/kg of water. The ratio of urine osmolality to plasma osmolality (U/Posm) is a good clinical index of the ability of the kidneys to concentrate or dilute glomerular filtrate.

  • A U/Posm ratio above 1 indicates that the kidneys are concentrating urine above plasma and glomerular filtrate. After water deprivation, the U/Posm of normal dogs may be 7 or higher.
  • A U/Posm ratio of about 1 indicates that water and solute are being excreted in a state that is isosmotic with plasma.
  • A U/Posm ratio markedly below 1 indicates that the tubules are capable of absorbing solute in excess of water (i.e., they are diluting glomerular filtrate).

In the next Diagnotes (Part 3) we will give you the chance to apply the concepts discussed in parts Parts 1 and 2 to clinical cases admitted to the veterinary teaching hospital.

Dr. Carl A. Osborne is the director of the Minnesota Urolith Center and a professor at the College of Veterinary Medicine at the University of Minnesota. Dr. Eugene Nwaokorie is pursuing a PhD at the University of Minnesota.


Source: DVM360 MAGAZINE,
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