Osmolality

Background

Osmolar Gap

• Primary determinants are sodium, glucose, BUN, chloride
• Osmolal gap
  • When measured minus calculated > 10
  • Suggests unmeasured, low molecular weight solutes

Values

• Calculated osmolarity = 2Na + Gluc/18 + BUN/2.8
  • Sodium multiplied by two to account for physiologic anions (chloride and bicarbonate)
  • Na in mEq/L, Glucose in mg/dL, BUN in mg/dL
  • Normal 280-295 mosmol/kg

DDx

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

Sources

• Emmett M et al. Serum osmolal gap. UpToDate. Aug 25, 2014. http://www.uptodate.com/contents/serum-osmolal-gap.