In a previous issue of dvm360 ("Urine specific gravity measurement and interpretation in veterinary medicine," May 2013), we discussed the variables influencing
normal specific gravity values. Here, we continue this review by discussing the pathophysiology of abnormal values.
Impaired urine concentration
The interpretation of urine specific gravity values of randomly obtained samples depends on knowledge of the patient's hydration
status and diet history, the plasma or serum concentration of urea nitrogen or creatinine, and knowledge of drugs or fluids
that have been administered to the patient. Urine volume and water consumption may also be helpful. In some instances, interpretation
may require serially performed evaluation of urine specific gravity on multiple samples. In others, evaluation of urine and
plasma osmolality is needed.
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If sufficient clinical evidence is present to warrant examination of the patient's renal function by determining the serum
concentration of creatinine or blood urea nitrogen, the urine specific gravity (or osmolality) should be evaluated at the
same time. Why? Because an adequately concentrated urine sample associated with an abnormal elevation in the serum creatinine
or urea nitrogen concentration suggests the probability of prerenal azotemia, whereas intrarenal azotemia is probable in patients
with elevated serum urea nitrogen and creatinine concentrations and less concentrated urine (Tables 1, 2 and 3).
Table 1: Differentiation of different forms of azotemia
Varying degrees of impaired ability to concentrate or dilute glomerular filtrate are a consistent finding in all forms of
primary renal failure but not all forms of renal disease. Because the kidneys have substantial functional reserve capacity,
impairment of their ability to concentrate or dilute urine may not be detected until at least two-thirds (dogs) or more (cats)
of the total population of nephrons has been damaged.
Table 2: Urine specific gravity values
Complete inability of the nephrons to modify glomerular filtrate typically results in formation of urine with a specific gravity
that is similar to that of glomerular filtrate (1.008 to 1.012). This phenomenon has commonly been called fixation of specific gravity. Once the ability to concentrate or dilute urine has been permanently destroyed, repeated evaluation of urine specific gravity
will not help in the evaluation of progressive deterioration of renal function. Therefore, serial evaluation of urine specific
gravity is of greatest aid in detecting functional changes earlier during the course of progressive primary renal failure
or in monitoring functional recovery associated with reversible renal diseases.
Table 3: Osmolality and urine specific gravity values for dog, cat and human urine
Total loss of the ability to concentrate and dilute urine (specific gravity = 1.008 to 1.012) often does not occur as a sudden
event but may develop gradually. For this reason, urine specific gravity values between about 1.007 to 1.029 in dogs and 1.007
to 1.039 in cats associated with azotemia are highly suggestive of primary renal failure, although, on occasion, hypoadrenocorticism
may induce similar findings (Tables 1, 2 and 3). Likewise, urine specific gravity values between about 1.007 to 1.029 in dogs
and 1.007 to 1.039 in cats that are clinically dehydrated but not azotemic are highly suggestive of primary renal failure
or other disorders that impair urine concentrating capacity (Table 4).
Table 4: Formation of dilute urine (urine specific gravity < 1.007, especially in patients that need to conserve water) may
represent an abnormality associated with several diseases including:
If nonazotemic patients have impaired ability to concentrate urine, investigate causes of pathologic polyuria. If you determine
the urine specific gravity or osmolality, it may allow you to determine whether a polyuric disorder characterized by water
(1.001 ± 1,006) or solute (±1.008 or greater) diuresis is probable.