Osmometry

Osmometry is the measurement of osmolality, the concentration of osmotically active particles in a solution, expressed as milliosmoles per kilogram of solvent (mOsm/kg). In biological fluids, osmolality reflects the number of solute particles regardless of their chemical identity and is a key factor governing water movement across cell membranes.

Osmometers determine osmolality by one of several physical principles. Freezing point depression osmometry measures how much the sample lowers the freezing point of water, converting that depression to osmolality using thermodynamic constants. Vapor pressure osmometry assesses the drop in vapor pressure caused by solutes, with osmolality proportional to the decrease in vapor pressure. Less common methods include other cryoscopic approaches. In clinical laboratories, freezing point osmometry is widely used for its speed and direct measurement of osmolality.

Clinically, osmolality is used to assess hydration status, guide management of electrolyte disorders, and evaluate kidney function. It is routinely measured in serum or plasma and in urine, and sometimes in cerebrospinal fluid. The osmolar gap, the difference between measured osmolality and calculated osmolality estimated from major solutes (commonly 2×Na+ plus glucose/18 plus BUN/2.8 in mOsm/kg), helps detect the presence of exogenous solutes such as alcohols or glycols when the gap is elevated.

Limitations include interference from temperature, sample handling, and the presence of nonvolatile or highly concentrated solutes; the calculated osmolality may misestimate true osmolarity if non-measured solutes or abnormal protein or lipid content are present. Osmometry complements other clinical measures in diagnosing osmolality-related disorders.