Measured osmolality

Osmolality can be measured on an analytical instrument called an osmometer

Osmometer

An osmometer is a device for measuring the osmotic strength of a solution, colloid, or compound.There are several different techniques employed in osmometry:...

. It works on the method of depression of freezing point

Freezing Point

Freezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...

.

Plasma osmolality is affected by changes in water content. In comparison, the plasma osmolarity is slightly less than osmolality, because the total plasma weight (the divisor

Division (mathematics)

right|thumb|200px|20 \div 4=5In mathematics, especially in elementary arithmetic, division is an arithmetic operation.Specifically, if c times b equals a, written:c \times b = a\,...

 used for osmolality) excludes the weight of any solutes, while the total plasma volume (used for osmolarity) includes solute content. Otherwise, one litre of plasma would be equivalent to one kilogram of plasma, and plasma osmolarity and plasma osmolality would be equal. However, at low concentrations, the weight of the solute is negligible compared to the weight of the solvent, and osmolarity and osmolality are very similar.

Technically, the terms can be compared as follows:

Origin	Source	Appropriate term
clinical laboratories	osmometer (freezing point depression osmometer Osmometer An osmometer is a device for measuring the osmotic strength of a solution, colloid, or compound.There are several different techniques employed in osmometry:... , or vapor pressure Vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...  depression osmometer Osmometer An osmometer is a device for measuring the osmotic strength of a solution, colloid, or compound.There are several different techniques employed in osmometry:... )	osmolality
bedside calculations	derived from lab data that was measured as solutions	osmolarity

Therefore, bedside calculations are actually in units of osmolarity, whereas laboratory measurements will provide readings in units of osmolality. In practice, there is almost negligible difference between the absolute values of the different measurements. For this reason, both terms are often used interchangeably, even though they refer to different units of measurement.

Human

Normal human reference range of osmolality in plasma is about 275-299 milli-osmoles per kilogram.

Nonhuman

Plasma osmolarity of some reptiles, especial those from a freshwater aquatic environment may be lower than that of mammals (eg < 260 mOsm/L) during favourable conditions. Consequently, solutions osmotically balanced for mammals (eg, 0.9% normal saline) are likely to be mildly hypertonic for such animals.
Many arid species of reptiles and hibernating uricotelic

Uricotelic

An Uricotelic organism produces uric acid as a result of deamination. An example of such an organisms include birds and insects. Other methods of deamination include Ammoniotelic and Ureotelic....

 species allow major elevations of plasma osmolarity (eg > 400 mOsm/L) that could be fatal to some mammals.

Clinical relevance

As cell membrane

Cell membrane

The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

s in general are freely permeable to water, the osmolality of the extracellular fluid

Extracellular fluid

Extracellular fluid usually denotes all body fluid outside of cells. The remainder is called intracellular fluid.In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma...

 (ECF) is approximately equal to that of the intracellular fluid (ICF). Therefore, plasma osmolality is a guide to intracellular osmolality. This is important, as it shows that changes in ECF osmolality have a great effect on ICF osmolality — changes that can cause problems with normal cell functioning and volume. If the ECF was to become too hypotonic, water would readily fill surrounding cells, increasing their volume and potentially lysing

Lysis

Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....

 them (cytolysis

Cytolysis

Cytolysis, or osmotic lysis, occurs when a cell bursts due to an osmotic imbalance that has caused excess water to move into the cell. It occurs in a hypotonic environment, where water diffuses into the cell and causes its volume to increase. If the volume of water exceeds the cell membrane's...

).

Osmolality of blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 increases with dehydration

Dehydration

In physiology and medicine, dehydration is defined as the excessive loss of body fluid. It is literally the removal of water from an object; however, in physiological terms, it entails a deficiency of fluid within an organism...

 and decreases with overhydration. In normal people, increased osmolality in the blood will stimulate secretion of antidiuretic hormone (ADH). This will result in increased water reabsorption, more concentrated urine

Urine

Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

, and less concentrated blood plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.

Increased osmolarity frequently occurs following illness due to chronic neurotoxic

Neurotoxin

A neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...

 diseases such as Lyme disease

Lyme disease

Lyme disease, or Lyme borreliosis, is an emerging infectious disease caused by at least three species of bacteria belonging to the genus Borrelia. Borrelia burgdorferi sensu stricto is the main cause of Lyme disease in the United States, whereas Borrelia afzelii and Borrelia garinii cause most...

.

Elevation may be associated with stroke mortality.

Calculated osmolarity (CO)

According to the international SI unit use the following equation :

Calculated osmolarity = 2 Na + Glucose + Urea ( all in mmol/L).

or

Calculated osmolarity = 2 Na + 2 K + Glucose + Urea ( all in mmol/L).

To calculate plasma osmolarity use the following equation (typical in the US):

• = 2[] + [Glucose]/18 + [ BUN
  Bun
  A bun is a small, usually sweet, bread. Commonly they are hand-sized or smaller, domed in shape, with a flat bottom. A bun can also be a savory bread roll similar to a bap or barmcake....
  
   ]/2.8 where [Glucose] and [BUN] are measured in mg/dL.

Simplifications are sometimes used:

• = 2[] + [Glucose]/20 + BUN/3 – 2

Osmolar gap (OG)

The osmolar gap is the difference between the measured osmolality and the calculated osmolarity. The difference in units is attributed to the difference in the way that blood solutes are measured in the laboratory versus the way they are calculated. The laboratory value measures the freezing point depression, properly called osmolality while the calculated value is given in units of osmolarity. Even though these values are presented in different units, when there is a small amount of solute compared to total volume of solution, the absolute values of osmolality vs. osmolarity are very close. Often, this results in confusion as to which units are meant. For practical purposes, the units are considered interchangeable. The resulting "osmolar gap" can be thought of as either osmolar or osmolal, since both units have been used in its derivation. This is acceptable because, as stated above, the absolute values of these units will be very close under clinical circumstances, as because the value is meant only as an estimate to help in the differential diagnosis of the clinical issue at hand.

Measured osmolality is abbreviated "MO", calculated osmolarity is abbreviated "CO", and the osmolality gap is abbreviated "OG".