Osmolarite

Osmolarite is a measure of the concentration of osmotically active particles in a solution. It expresses the amount of osmoles per liter of solution and is commonly reported in milliosmoles per liter (mOsm/L). In English usage, osmolarite corresponds to osmolarity, a quantity that reflects the osmotic pressure a solution would exert across a semipermeable membrane.

Osmolarite and osmolality are related but not identical. Osmolarite is per liter of solution, whereas osmolality is per kilogram of solvent. In dilute aqueous solutions and typical biological fluids, osmolarite is often similar to osmolality, but temperature and solution density can cause differences. The plasma osmolarite is typically about 275–295 mOsm/L.

Contributors to osmolarite include major electrolytes (primarily sodium and its accompanying anions), potassium, glucose, and urea, along with other solutes such as alcohols or metabolites present in the fluid. Clinically, osmolarite helps assess fluid balance, hydration status, and electrolyte disturbances. A useful concept is the osmolar gap, defined as the difference between the measured osmolality and the calculated osmolarite from the major solutes; a significant gap suggests unmeasured osmoles such as ethanol, methanol, ethylene glycol, or mannitol.

Measurement techniques include osmometry, with methods such as freezing-point depression osmometry and vapor pressure osmometry. While osmolarite provides a concise summary of solute concentration, it must be interpreted alongside volume status, renal function, and other laboratory data.