milliosmolarity

Milliosmolarity, usually written as milliosmoles per liter (mOsm/L), is a measure of the osmolar concentration of a solution. It reflects the total number of osmotically active particles per liter of solution, accounting for solute dissociation. In dilute biological fluids, milliosmolarity approximates osmolarity (Osm/L) and is closely related to osmolality (mOsm/kg), though the two are not identical in all situations.

Calculation and interpretation: mOsm/L is the sum of contributions from all solutes, using the van’t Hoff factor to account for dissociation. If C_i is the molar concentration of solute i and i is its van’t Hoff factor, then osmolarity is Σ(i × C_i) Osm/L, and milliosmolarity is this value multiplied by 1000 to convert to mOsm/L. For example, NaCl at 0.154 M (i ≈ 2) yields about 0.308 Osm/L or 308 mOsm/L. Nondissociated solutes contribute roughly their concentration in mOsm/L.

Clinical context: Plasma milliosmolarity is typically in the range of about 275–295 mOsm/L. Intravenous solutions are described by osmolarity: isotonic solutions are roughly 275–295 mOsm/L (normal saline is about 308 mOsm/L), hypotonic solutions are below ~275 mOsm/L, and hypertonic solutions exceed ~295 mOsm/L. Osmolarity can be measured directly with an osmometer or estimated from solute concentrations.

Osmolar gap: Clinically, measured osmolality (mOsm/kg) can be compared with calculated osmolarity to yield an osmolar gap, which may indicate the presence of unmeasured solutes such as alcohols or toxins.

Overall, milliosmolarity provides a quantitative sense of the solute burden in a solution and is widely used in physiology, medicine, and laboratory science.