osmometer

An osmometer is a laboratory instrument used to measure the osmotic concentration of a solution, reported as osmolality in milliosmoles per kilogram (mOsm/kg). Osmolality reflects the number of dissolved particles that contribute to osmotic pressure and is commonly used to assess body fluids and prepared solutions.

Most osmometers operate by one of two principal methods. Freezing-point depression (cryoscopic) osmometry determines how much a solution lowers the freezing point of a solvent; the magnitude of the depression is proportional to osmolality. Vapor-pressure osmometry measures the reduction in solvent vapor pressure caused by dissolved solutes. A key distinction is that freezing-point methods are sensitive to colligative properties, while vapor-pressure methods assess solute concentration via vapor dynamics.

Devices are typically benchtop instruments with micro-sampling capability, often requiring only microliters to a few hundred microliters of sample. They are calibrated with standard osmolal solutions and then provide an osmolality readout, usually in mOsm/kg. In clinical practice, reference ranges include serum osmolality of about 275–295 mOsm/kg, while urine osmolality is highly variable and hydration-dependent.

Interpretation often involves calculating the osmolal gap, defined as the difference between measured osmolality and calculated osmolality using common solutes: Osm_calc ≈ 2[Na+] + Glucose/18 + BUN/2.8 (in mOsm/kg). An elevated osmolal gap can indicate the presence of unmeasured substances such as alcohols or glycerol.

Advantages of osmometry include rapid results from small samples and direct measurement of osmotic pressure. Limitations include potential interference by volatile solutes in some methods and the need for careful calibration and temperature control.