Plasma osmolality

Plasma osmolality is a measure of the concentration of substances such as sodium, chloride, potassium, urea, glucose, and other ions in human blood. It is calculated as the osmoles of solute per kilogram of solvent. Normal osmolality in plasma is about 280 - 303 milli-osmoles per kilogram. It is affected by changes in water content.

To calculate plasma osmolarity (osmolality) use the following equation:
P_osm = 2 [Na⁺] + [Glucose] /18 + [ BUN ] /2.8

The above equation holds true if the glucose & ureas blood chemistry units are given as: mg/dL.

If the units are given as: mmol/l; then the calculated osmolality is given by:

P_.OSM = 2 [NA] + 2 [K+] + [Urea] + [Glucose]

Osmolality can be measured on an analytical instrument called an Osmometer. It works on the method of depression of freezing point.

The "Osmolar gap" is the diffence between the measured osmolality and the calculated osmolality.

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 affect 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).