How specific is urine specific gravity?

How specific is urine specific gravity?

An in-depth look at which urinary concentrating ability test might be best in your patients. (Part one of a four-part series.)

Suppose you are evaluating a persistently polyuric golden retriever. The serum chemistry profile results are normal. Evaluation of a urine sample reveals 3+ proteinuria (determined by reagent strip) and no abnormalities in the urine sediment. Which of the following test results obtained from a refrigerated urine sample (40 F) most accurately reflect urine concentrating capacity?
1. Urine osmolality = 1,400 mOsm/kg
2. Urine specific gravity = 1.025 measured by urinometer
3. Urine specific gravity = 1.014 measured by a reagent test
4. Urine specific gravity = 1.017 measured by a refractometer

What urine osmolalityand urine specific gravity measure

The kidneys excrete unwanted solute (urea, creatinine, minerals and other metabolic garbage) in a volume of water that is not required to maintain homeostasis. Elimination of unwanted solutes and water is the result of carefully regulated glomerular filtration, tubular reabsorption and tubular secretion.

Measurement of urine osmolality, either directly by osmometry or indirectly by evaluation of urine specific gravity, is the primary method used to evaluate the kidneys' response ability to concentrate (remove water in excess of solute) or dilute (remove solute in excess of water) urine according to varying needs. Thus, it is an index of tubular reabsorption. Knowledge of urine osmolality or specific gravity is extremely helpful when attempting to differentiate the underlying cause of polyuria and when localizing the pathophysiologic mechanisms of azotemia.

Another major indication for routine evaluation of urine specific gravity involves interpretation of other test results that are part of the complete urinalysis. Interpretation of other urinalysis test results are dependent on knowledge of specific gravity (or osmolality) since the specific gravity value provides information regarding the ratio of solutes to solvent (water). Semiquantitative interpretation of other test results is unfeasible in randomly obtained urine samples without knowledge of specific gravity. Consider proteinuria as an example: Does 2+ proteinuria at specific gravity of 1.010 reflect an equal or greater loss of protein than a 2+ proteinuria at 1.050? 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.