How specific is urine specific gravity? - DVM
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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.)


Urine specific gravity

The density of a substance is the ratio of its mass (weight) to its volume. Density of urine reflects the total mass (which is related to weight and, thus, gravity) of all solutes per unit volume of solution. Stated another way, urine specific gravity is the ratio of the density (or weight) of urine to the density (or weight) of an equal volume of distilled water, both measured at the same temperature:
Specific gravity = Density of urine / Density of water

The specific gravity of water is 1.000 under conditions of standard temperature and pressure. If the density of urine were equal to the density of water, the specific gravity value would be 1.000. However, it is physiologically impossible for the kidneys to excrete pure water. Urine is more dense than water because it is composed of water and various solutes of different densities. Therefore, urine always has a specific gravity greater than 1.000.

Because specific gravity is a measurement of density, it is affected by the number of particles of solute present. Unfortunately, it is also affected by the molecular weight of each solute present. Therefore, there is only an approximate relationship between specific gravity and total solute concentration.

Each species of solute has its own characteristic effect on the specific gravity of urine. Urine samples having equivalent numbers of solute molecules per unit volume may have different specific gravity values if different mixtures of solutes are present.

For example, equal numbers of molecules of sodium chloride, albumin and glucose all have a different quantitative effect on specific gravity. Looking at this illustration in a slightly different context, adding 0.147 g of sodium chloride, 0.27 g of glucose or 0.4 g of albumin to 100 ml of urine will increase urine specific gravity by 0.001.

Although the weight of urine remains constant regardless of its temperature, the density of urine decreases with an increase in temperature. Conversely, the density of urine increases with a decrease in temperature. Therefore, for more precise work, the temperature of urine should be compared with the reference temperature of the instrument used to determine specific gravity.

Urinometers to measure urine specific gravity

A urinometer consists of a weighted glass bulb attached to a cylindrical stem designed to measure the specific gravity of urine. A scale calibrated in specific gravity units is etched on the surface or placed inside the cylindrical stem. Urinometers are calibrated at a reference temperature (usually close to room temperature). When placed in a solution such as urine, the urinometer displaces a volume of urine equal to its weight. The more solute that urine contains, the less volume the urinometer displaces. The fluid level is read at the bottom of the meniscus where urine intersects with the urinometer specific gravity scale.

Compared with refractometry and osmolality, the precision of urinometers is not great. One reason already mentioned is that urinometer measurements vary with temperature. It also may be difficult to read the meniscus. Another disadvantage is that there is a tendency for urinometers to drag against the side of narrow deep cylinders containing the urine sample. This may result in erroneous specific gravity values. In addition, a relatively large volume of urine (i.e., 5 to 15 ml or more depending on the size of the urinometer and the size of the container) is required in which to float this instrument.


Source: DVM360 MAGAZINE,
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