Calculate serum osmolarity and determine the osmolar gap.
Normal serum osmolarity = 285 to 295
(Na+/2 + BUN/2.8 + Glu/18 + BAL/4.6)
Normal osmolal gap = -14 to 10
The osmolal gap will be elevated if toxic alcohols (methanol, ethylene glycol, isopropyl alcohol, propylene glycol) are present. However, as the parent compounds are metabolized into their toxic metabolites, the osmolal gap normalizes and the anion gap elevates (except isopropyl alcohol, which causes ketosis d/t acetone).