Osmolality

Background

• Primary determinants are sodium, glucose, BUN, chloride
• Osmolal gap
  • When measured minus calculated > 10
  • Suggests unmeasured, low molecular weight solutes
    • EtOH
    • Ethylene glycol
    • Methanol
    • Isopropyl alcohol
    • Diethylene glycol
    • Propylene glycol
    • Mannitol
    • Glycerol
    • Smaller gaps possible in:
      • Ketoacidosis
      • Lactic acidosis
      • Chronic kidney disease

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

Sources

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