Plasma osmolality

Measured osmolality (MO)
Osmolality can be measured on an analytical instrument called an osmometer. It works on the method of depression of freezing point.

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 used for osmolality) excludes the weight of any solutes, while the total plasma volume (used for osmolarity) includes solute content. Otherwise, one liter 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:

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

Clinical relevance
As cell membranes in general are freely permeable to water, the osmolality of the extracellular fluid (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 them (cytolysis).

Osmolality of blood increases with dehydration 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, 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 diseases such as Lyme disease.

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 ]/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".