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  • osmolality is miliosmoles per kg of weight
  • osmolarity is miliosmoles per L of volume
  • Osmometer measures osmolality but it reports osmolarity
  • Calculated osmolarity
    • 2 Na + glucose/18 + BUN/2.8
      • This is based on formula weight
  • osmolar gap: measured osmolarity - calculated osmolarity
    • Normal osmolar gap is < 10 mosm/L

Increased Osmolar Gap with Acidosis

  • Methanol
  • Ethylene glycol
  • propylene glycol

Increased Osmolar Gap without Acidosis

  • Ethanol
  • Isopropanol
  • Acetone
  • Mannitol
  • Sorbitol

Normal Osmolar Gap

  • Lactate
  • Uremia
  • Salicylate
  • Ketoacids

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