Plasma osmolality

Plasma osmolality measures the body's electrolyte-water balance. There are several methods for arriving at this quantity through measurement or calculation.

Osmolality and osmolarity are measures that are technically different, but functionally the same for normal use. Whereas osmolality (with an “ℓ”) is a measure of the osmoles (Osm) of solute per kilogram of solvent (osmol/kg or Osm/kg), osmolarity (with an “r”) is defined as the number of osmoles of solute per liter (L) of solution (osmol/L or Osm/L).

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

Osmolarity is affected by changes in water content, as well as temperature and pressure. In contrast, osmolality is independent of temperature and pressure. For a given solution, osmolarity is slightly less than osmolality, because the total solvent weight (the divisor used for osmolality) excludes the weight of any solutes, whereas the total solution 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 (below about 500 mM), the mass of the solute is negligible compared to the mass 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.

Normal human reference range of osmolality in plasma is about 285-295 milli-osmoles per kilogram.

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