In the March issue of dvm360 ("How specific is urine specific gravity?"; see the related link below), we asked several questions about urine specific
gravity, such as what does urine specific gravity measure? This month, we continue to investigate whether urine specific gravity
is really specific.
But first, let us review some of the major points we covered previously. The kidneys excrete unwanted solute (urea, creatinine,
minerals and other metabolic garbage) in a volume of water that is not itself required to maintain homeostasis. Elimination
of unwanted solutes and water is the result of carefully regulated glomerular filtration, tubular reabsorption and tubular
secretion. Serum urea and serum creatinine are predominantly dependent on the glomerular filtration rate (GFR) and, therefore,
are used to measure the GFR. So what does urine specific gravity measure? Urine specific gravity is used as a clinical index
of tubular function.
Methods of measurement
Measurements of urine specific gravity made by osmometers, refractometers, urinometers and reagent strips are related but
not interchangeable. Measurement of urine osmolality provides information that is more closely related to renal concentrating
capacity than does specific gravity or refractive index. Osmometers provide more accurate assessment of osmolality of individual
urine samples than refractometers or urinometers but are relatively expensive.
As mentioned, urine specific gravity is a direct but not proportional function of the number of solute particles in urine.
Urine specific gravity varies with the kind of solute present, whereas urine osmolality is independent of the types of solute
present. Therefore, urine specific gravity provides only an estimation of osmolality. Indirect measurement of urine specific
gravity by refractometry is useful as a screening test of renal function. Urine osmolality measured with an osmometer should
be used for patients with undiagnosed persistent polyuria when errors in assessment of renal function are of significant consequence.
Vapor pressure osmometers are preferable to freezing point osmometers when assessing urine samples with high osmolality.
Evaluation of urine specific gravity is essential when interpreting test results of the complete urinalysis. Refractometers
are recommended over urinometers for determination of urine specific gravity because they provide more reproducible results,
require a small sample size, are temperature-compensated and are technically easy to use.
Measurement of urine osmolality, either directly by osmometry or indirectly by evaluation of urine specific gravity, is the
primary method used to evaluate the kidney's "response ability" to concentrate (remove water in excess of solute) or dilute
(remove solute in excess of water) urine. Thus, evaluation of urine osmolality or specific gravity is an index of tubular
reabsorption. Knowledge of urine osmolality or specific gravity is also extremely helpful when attempting to differentiate
among the underlying causes of polyuria and when localizing the pathophysiologic mechanisms of azotemia (Table 1).
Table 1 Differentiation of different forms of azotemia
Routine evaluation of urine specific gravity involves interpretation of tests that are part of the complete urinalysis. Interpretation
of other test results of the urinalysis depend on knowledge of specific gravity (or urine osmolality) since these data provide
information regarding the ratio of solutes to solvent (water). Tests of routine urinalyses are typically performed on a relatively
small sample of urine without regard to the rate of formation of urine or total urine volume. Semiquantitative interpretation
of results is unfeasible in such samples without knowledge of specific gravity.
Consider proteinuria as an example. Does 2+ proteinuria at a specific gravity of 1.010 reflect an equal or greater loss of
protein than a 2+ proteinuria at 1.050? The answer is obvious—there is more protein in the less concentrated sample. The same
concept is applicable to interpretation of positive test results for glucose, ketones, bilirubin, occult blood and constituents
in urine sediment.
Another indication for evaluation of urine specific gravity or osmolality is as an aid for monitoring the patient's fluid
balance, especially during therapy with parenteral fluids.