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Cornell Pediatrics

Serum Osmolality

Calculate the predicted serum osmolality given the serum sodium, glucose and blood urea nitrogen.

Input Serum Na  mEq/L
Input Serum Glucose  mg/dL
Input BUN  mg/dL
 
 
Predicted Serum Osmolality  =   mOsm/kg

 

The normal range of serum osmolality is 285-295 mOsm/kg. The measured osmolality should not exceed the predicted by more than 10 mOsm/kg. A difference of more than 10 mOsm/kg is considered an osmolal gap. Causes for a serum osmolal gap include mannitol, ethanol, methanol, ethylene glycol and other compounds in very high concentration, usually small molecules and often toxins. The propylene glycol in lorazepam may cause hyperosmolarity and sometimes hyperosmolar coma in some patients, particularly when the lorazepam is used as a continuous infusion.

The equation is:

Osmolality = 2 * Na + 
Glu
 + 
BUN
18
2.8

 

Created: January 7, 2000
Revised: October 3, 2000

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