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

Differential Diagnosis

• EtOH
• Ethylene glycol
• Methanol
• Isopropyl alcohol toxicity
• Diethylene glycol
• Propylene glycol
• Mannitol
• Glycerol
• Smaller gaps possible in:
  • Ketoacidosis
  • Lactic acidosis
  • Chronic kidney disease

See Also

• Anion Gap and Osmolar Gap (High)
• Anion Gap (High)
• Toxidromes

External Links

• MDCalc - Serum Osmolality/Osmolarity
• MDCalc - Anion Gap

References

Authors

Kevin Lu