Osmolality
Osmolality is an estimation of the osmolar concentration of plasma and is proportional to the number of particles per kilogram of solvent; it is expressed as mOsmol/kg (the SI unit is mmol/kg but mOsmol/kg is still widely used). This is what is used when values are measured by a laboratory. Osmolality is measured by clinical laboratories using an osmometer - either a freezing point depression osmometer or a vapour pressure depression osmometer. The normal osmolality of extracellular fluid is 280-295 mOsmol/kg.