Osmolality in plasma 280-300 mosmol/kg H2O
Osmolaltiy is an expression for the osmotic effect of dissolved particles per kg of solute. In the extracellular fluid, sodium is the most important osmotic factor. The osmolality is regulated by regulating concentrations of sodium. Urea and glucose also contribute a little.
The measured osmolality must be interpreted in relation to the calculated osmolality:
Calculated osmolality = 2 * S-Natrium + S-Glucose + S-Urea
If the patient has a normal measured osmolality and a calculated osmolality that is at least 10 mosomol/kg H2O lower, it is called pseudohyponatremia. In the clinical context, the term is often used for conditions where fluid is drawn from the intracellular space to the extracellular space. This can happen in severe hyperglycemia and leads to the normalisation of the osmolaltiy and reduced concentration of all the dissolved substances in the serum or plasma that are not in an equilibrium between the intracellular and extracellular rooms. This is easiest to measure for sodium, and thereof the slightly misleading term, pseudohyponatremia. In the laboratory, one can see falsely low sodium when the concentrations of proteins or lipids in plasma are elevated. Normally, proteins and lipids take up little space (< 5 %), but when sick, this can increase significantly and lead to the measured sodium per volume unit being falsely low.
If the measured osmolality in the plasma or serum is elevated and at least 10 mosmol/kg H2O higher than the calculated osmolality, intoxication with low-molecular compounds is indicated, such as ethanol, methanol, isopropanol or ehtylene glycol.
Read more about osmolality in The handbook on medical biochemistry