Osmotic pressure is the pressure applied to a solution to prevent the inward flow of water across a semipermeable membrane. The osmotic pressure of a solution depends on the number of particles in solution. For an ideal solution the osmotic pressure is directly proportional to its molality (number of moles of solute per kilogram of the solvent).

Osmolality of a solution correspond to the molality of an ideal solution containing nondissociating solutes. It is expressed in osmoles per kilogram of solvent (Osmol per kg). Osmolality is typically determined by measuring freezing point depression. 

For water, 1 Osmol of a solute added to 1 kg lowers the freezing point by 1.86°C.

The osmolality of blood ranges between 285 and 310 mOsmol per kg.

Osmolarity of a solution is a theoretical quantity expressed in osmoles per L (Osmol per L). Osmolarity cannot be measured but is calculated theoretically from the experimentally measured value of osmolality.

DB Lab can measure the osmolality of a solution by the freezing point depression technique according to the European Pharmacopeia 2.2.35 and the U.S. Pharmacopeia chapter <785>.