This is an estimation of the osmolar concentration of plasma derived from the measured Na, K, urea and glucose concentrations. The osmolarity is unreliable in various conditions e.g. psuedohyponatraemia such as hyperlipidaemia in nephrotic syndrome, or hyperproteinaemia.
Osmolarity is proportional to the number of particles per litre of solution (i.e. in the case of plasma, per litre of whole plasma). This contrasts with osmolality which is proportional to the number of particles per kg of solvent.
Last reviewed 01/2018