Lab

Osmolar Gap

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Osmolar Gap, Osmolal Gap

  • See Also
  1. Sodium and Water Homeostasis
  2. Hyponatremia
  3. Hypernatremia
  4. Serum Sodium
  5. Total Body Sodium Deficit
    1. Includes Total Body Water and Free Water Deficit Calculations
  6. Serum Osmolality
  7. Urine Sodium
  8. Fractional Excretion of Sodium (FENa)
  • Calculation
  1. Osmolar Gap = Serum Osms - Calculated Osmolality
  2. Normal Gap <10 to 15 mOsm/kg water
  • Causes
  • Elevated Osmolal Gap
  1. Calculated Serum Osmolality > Measured Serum Osms
    1. Lab Error
    2. Mathematical error
  2. Measured Serum Osms > Calculated Serum Osmolality
    1. Decreased serum water (Calculated osmolality low)
      1. Hyperproteinemia
      2. Hypertriglyceridemia
    2. Presence of unmeasured Osms (Mnemonic: ME DIE)
      1. General
        1. Unmeasured osmoles are membrane permeant solutes
        2. Known as ineffective osmoles
        3. Do not affect water balance between ICF and ECF
      2. Methanol (decreased Blood pH)
      3. Ethylene Glycol (decreased Blood pH)
      4. Diuretics (Mannitol)
      5. Isopropanol or Isopropyl Alcohol (acetone detected)
      6. Ethanol
      7. Sorbitol
      8. Glycerol
      9. Acetone
      10. Ether trichloroethane
      11. Paraldehyde
  1. Precautions
    1. Note that Isopropanol (Isopropyl Alcohol) does not increase Anion Gap, but does increase Osmolal Gap
  2. Eythylene glycol
  3. Methanol
  4. Alcoholic Ketoacidosis
  5. Diabetic Ketoacidosis
  • References
  1. Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]