To the Editor.
—The Council Report1 on dehydration warrants several practical caveats. While touching briefly on the influence of serum urea nitrogen, glucose, alcohol, and hyperlipidemia on measured serum osmolality, it fails to indicate, in practice, how these may complicate the formula given for free water deficit (FWD).Since measured serum osmolality is increased by the presence of the osmotically active agents urea and alcohol, which are relatively freely diffusible across cell membranes, this measurement is of little value in assessing FWD when these are present. The information one is seeking is captured, conceptually, in the term "effective osmolality" (Eosm), which we2 have arbitrarily calculated as follows:(For values in Système International (SI) units, given as mmol/L, simply eliminate the conversion factor of 18 for serum glucose. The conversion factor for sodium and potassium is 1.)Normal Eosm is 290±5 mOsm/L. The average value of Eosm of 290
Matz R. Dehydration in Older Adults. JAMA. 1996;275(12):911-912. doi:10.1001/jama.1996.03530360021030