1. What is the difference between urine osmolality and specific gravity?
Specific gravity is a common part of a urinalysis. It evaluates the weight of solids in water. Osmolality and specific gravity usually change in parallel to each other. When large and heavy molecules (such as glucose and protein) are present in the urine, however, the results will diverge. Specific gravity will be increased more, due to the weight of the molecules, while urine osmolality will be increased less, reflecting the number of molecules. Generally, osmolality is considered a more exact measurement of urine concentration than specific gravity.
2. What is the difference between osmolality and osmolarity?
Osmolality measures the concentration of solutes in a fluid by looking at the number of particles per weight (kilogram) of fluid. Osmolarity evaluates the number of particles per volume (liter) of fluid. With dilute fluids, they are essentially the same, but differences will be more noticeable at higher concentrations and care must be taken when comparing them as the units for osmolality and osmolarity are not the same.
This article was last reviewed on October 25, 2013. | This article was last modified on April 10, 2015.
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