Osmolal or Osmolar Gap

Definitions

• Osmolarity = the total number of particles in solution per liter
• Osmolality = the number of particles per kilogram of solvent

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)

• Normal = 275-295

Osmolal Gap

• Osmolal Gap = measured osm - calculated osm
  • Normal = (-)14-10 mOsm/L

DDX

1. Alcohols
   1. Methanol
   2. Ethylene glycol
   3. Isopropyl
2. Sugar
   1. Glycerol
   2. Manitol
3. Ketones
   1. Acetone

See Also

• Anion Gap and Osmolar Gap (High)