Specific Gravity of Urine
Urine specific gravity (USG) and osmolality are measures of the solute concentration in urine and are used to assess the ability of the renal tubules to concentrate or dilute the glomerular filtrate.
The diagram and notes below detail how the kidney concentrates urine.
|Schematic representation of the passive urinary concentrating mechanism|
The thin ascending limb in the inner medulla and the thick
ascending limb in the outer medulla and first part of the distal
tubule are permeable to NaCl but impermeable to water (as indicated
by the thickened lining).
- In the water-impermeable thick ascending
limb, absorption of NaCl via the NaK2Cl carrier renders the
tubular fluid dilute and the outer medullary interstitium
hyperosmotic. Urea is poorly absorbed and is retained in the
- Water is resorbed down the osmotic
gradient established in the outer medulla in the last part
of the distal tubule and collecting ducts (the latter under
the influence of ADH).
- In the inner medulla, water and urea
are absorbed (under the action of ADH) from the collecting
duct. Urea contributes substantially to the medullary interstitial
- The high concentration of urea in the
medullary interstitium osmotically extracts water from the
solute-impermeable descending limb, thus concentrating NaCl
in the descending-limb fluid.
- When this NaCl-rich fluid enters the
NaCl-permeable (water-impermeable) thin ascending limb, NaCl
is absorbed passively along its concentration gradient, producing
a relatively dilute renal tubule fluid.
The dilute renal tubular fluid will then become concentrated
when ADH stimulates water absorption in the collecting tubule
(step 3). The degree to which this water will be absorbed
(and how concentrated the urine will be) depends on the ability
of the tubule to create and maintain a hypertonic medullary
interstitium (for more information on this, see the countercurrent
Urine specific gravity is a measurement of the density of urine compared to pure water. For routine clinical purposes, USG is
determined using a refractometer (refractive index generally correlates well with USG). The USG is influenced by the number of molecules in urine, as well as their molecular weight and size, therefore it only approximates solute concentration. It is also affected by temperature, with urine density decreasing (lower USG) with increasing temperatures. The presence of large amounts of protein and glucose will alter the USG and should be considered when interpreting USG results. A concentration of 1 g/dL of the following substances in urine will increase the USG as follows:
Increase in USG
Urine osmolality is directly related to the number of particles in solution and is unaffected by molecular weight and size. Osmolality can be measured by freezing point depression (technique used at Cornell University) and changes in vapor pressure. Urine osmolality can be approximated from the USG, by multiplying the last 2 digits of the USG by 36.
The interpretation of several urine chemical parameters, such as protein and bilirubin, is also influenced
by the specific gravity of the specimen. In addition, urinary constituents (erythrocytes,
leukocytes and casts) can lyse in dilute urine (USG < 1.008), affecting
interpretation of the urine sediment results.
Knowledge of urinary solute concentration is essential for proper
interpretation of urea and creatinine, which are indicators of
glomerular filtration rate. A wide range of USG is possible in healthy euhydrated animals, however animals that are dehydrated, hypovolemic or have decreased effective blood circulating volume should be conserving water (and trying to reconstitute effective blood volume) and thus concentrating their urine. Thus, in the setting of azotemia or an increased urea and/or creatinine, the USG is used to determine if concentrating ability is adequate and is very useful for identifying the
cause of azotemia.
Interpretation: Indicated below are guidelines for interpreting the USG in animals. Note that different cut-offs for "adequate" concentrating ability and isosthenuria are reported in the literature. Given below are the ones used here at Cornell University.
- "Adequate" USG: The "adequate" USG or concentrating ability column is used specifically in azotemic animals. In this context, an "adequate" USG indicates the existence of sufficient functional nephrons to adequately concentrate the urine and to prevent development of azotemia, provided that renal blood flow is sufficient and that the ability of those nephrons to concentrate urine is not impaired by other factors, such as medullary solute washout (for more information, refer to countercurrent exchange mechanism). Thus, an adequate USG in an azotemic animal usually indicates a pre-renal azotemia.
- "Inadequate" USG: Azotemic animals with In nephropathies characterized by progressive loss of of functional nephrons, the ability to concentrate urine is compromised when about 2/3 of the nephron mass is lost. Clearance of nitrogenous waste products sufficient to prevent azotemia, however, persists until roughly 3/4 of functional nephrons are lost.
Therefore, if azotemia is due to loss of nephron mass (>3/4 loss, i.e., renal failure), ability to concentrate urine will have already been lost (i.e. the USG will be less than "adequate" for that species). An exception to this occurs in cats, in which glomerular disease (and azotemia) can precede loss of concentrating ability. Thus, an "inadequate" USG in an azotemic animal is compatible with renal disease and a renal azotemia UNLESS there are other factors that impair the ability of the kidney to concentrate, including decreased hypertonicity of the medullary interstitium (e.g. hyponatremia, low urea from liver disease, and polyuria, all of which lead to medullary solute washout) and inhibition of ADH action (e.g. hypercalcemia, hypokalemia, corticosteroids).
- Isosthenuria: In a primary renal azotemia, the kidney cannot concentrate or dilute urine, so there is often a fixed (constant) isosthenuric USG, i.e. USG of 1.008-1.012. However, in renal disease, the total loss of renal tubule function occurs gradually, therefore USGs between isosthenuric and "adequate" ranges in animals, with dehydration and/or azotemia, are highly suggestive of primary renal failure.
- Hyposthenuria: This indicates that the kidney can dilute the urine but is unable to concentrate, i.e. proximal renal tubule and loop of Henle function is retained but the tubules are unresponsive to ADH, either from a primary ADH deficiency (central diabetes insipidus) or renal tubule disease (nephrogenic diabetes insipidus).
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