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.)
Mar 01, 2013
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?
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.
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.