Serum Osmolality


Serum Osmolality, Calculated Serum Osmolality, Serum Osms, Blood Osmolarity, Extracellular Fluid Tonicity, Tonicity Disorder

  • Definitions
  1. Osmolality
    1. Total solute concentration in a fluid compartment
  2. Tonicity
    1. Concentration of osmoles in a compartment that generate osmotic pressure gradient
    2. Tonicity impacts water traversal from one compartment to another
    3. Only solutes that cannot move freely between compartments generate an osmotic gradient
      1. Blood Urea Nitrogen (BUN) does not affect tonicity, but does influence osmolality
      2. BUN crosses easily between compartments and does not generate an osmotic gradient
    4. Extracellular Fluid Tonicity is based on only a few effective osmoles (of which Sodium is key)
      1. Sodium concentration (primary)
      2. Glucose (severe Hyperglycemia in Diabetes Mellitus)
      3. Mannitol or Sorbitol (used to decrease Intracranial Pressure in closed Head Trauma)
  • Interpretation
  • Normal
  1. Serum Osmolality: 280-300 mOsm/(kg water)
  • Calculation
  • Calculated Serum Osmolality
  1. Serum Osms = 2*Na + Glu/18 + BUN/2.8 + Et/4.6 + Man/18
  2. Annotation
    1. Where Na is Serum Sodium
    2. Where Glu is Serum Glucose
    3. Where BUN is Blood Urea Nitrogen
    4. Where Et is Blood Alcohol Level
    5. Where Man is Mannitol
  • Causes
  • Decreased Serum Osmolality
  1. SIADH
  2. Hyponatremia
  3. Overhydration