Osmolal or Osmolar Gap
Definitions
Osmolarity = the total number of particles in solution per liter
Osmolality = the number of particles per kilogram of solvent
Background
Serum osmolality should be measured by the freezing-point depression method rather than by the vapor pressure method, since the latter technique will underestimate the concentrations of volatile alcohols (methanol, ethylene glycol, and isopropanol).
Calculations
Calculated Osm
Calculated Osm = 2 x Na = (glu/18) + (BUN/2.8) + (ethanol/4.6) + (methanol/2.6) + (ethylene glycol/5) + (acetone/5.5) + (isopropanol/5.9)
Osmolal Gap
Osmolal Gap = measured osm - calculated osm
(Normal = (-)14-10 mOsm/L)
Estimating Toxic Alcohol Concentration
Toxic alcohol (mg/dL) = Osmolal gap x Conversion Factor
^Parent compounds are responsible for the osmolar gap while their metabolites are responsible for anion gap changes
DDX
Increased)
- Alcohols
- methanol
- ethylene glycol
- isopropanol
- Sugar
- glycerol
- manitol
- Ketones
- acetone