calculated or estimated osmolality and the osmotic gap
this is work in progress - much has been written about the utility, or the lack thereof, of calculating this value.
the formulae used to derive it vary somewhat, but are similar.
all are based on the assumption that serum osmolality can be estimated based
on the the known kation concentrations plus urea and glucose. each kation is supposed to be balanced by an almost equal number of
osmotically active anions.
for the time being this programme uses the formula
estimated osmolality =
2*([Na] + [K]) + [gluc] + [urea] + [ethanol or other measured substances].
(please, do not register anionic substances like hydroxybutyrate, because their osmotic effect is already accounted for by the
corresponding kation!)
this value is then substracted from the osmolality as measured by the freezepoint method. any difference bigger than
10 mosmol/l is interpreted as indicating the presence of unknown smallmolecular weight solutes, typically methanol, ethanol or glycole.
for some reasons the Norwegians have had several incidents of mass methanol intoxications the last years, which gave them ample
opportunity for testing this, and finding it useful.
(Hovda et alii, ICM 2004, 30(9):1842, anion and osmolal gaps in the diagnosis of methanol poisoning).
