Acid Base Balance Series: What is the 'osmolar gap'?

NB: 'Osmolar gap' has several alternative names: 'osmol gap', 'osmole gap', 'osmolarity gap' & 'osmolal gap'; these all refer to the same thing. For consistency, the term "osmolar gap" is used exclusively through this book.

What is the 'osmolar gap'?

Definitions An osmole is the amount of a substance that yields, in ideal solution, that number of particles (Avogadro’s number) that would depress the freezing point of the solvent by 1.86K Osmolality of a solution is the number of osmoles of solute per kilogram of solvent. Osmolarity of a solution is the number of osmoles of solute per litre of solution.

So osmolality is a measure of the number of particles present in a unit weight of solvent. It is independent of the size, shape or weight of the particles. It can only be measured by use of a property of the solution that is dependent on the particle concentration. These properties are collectively referred to as Colligative Properties. Osmolality is measured in the laboratory by machines called osmometers. The units of osmolality are mOsm/kg of solute
Osmolarity is calculated from a formula which represents the solutes which under ordinary circumstances contribute nearly all of the osmolality of the sample. There are many such formulae which have been used. One widely used formula for plasma which is used at my hospital is:

Calculated osmolarity = (1.86 x [Na⁺]) + [glucose] + [urea] + 9

Note regarding units: For the above equation, all concentrations are in mmol/l, and not mg/100mls. The result will then be in mOsm/l of solution. This equation is often expressed differently in North America where glucose & blood urea nitrogen (BUN) are reported in