Serum Osmolality

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

The normal range of serum osmolality is 285-295 mOsm/L. The measured osmolality should not exceed the predicted by more than 10 mOsm/L. A difference of more than 10 mOsm/L is considered an osmolal gap. Causes for a serum osmolal gap include mannitol, ethanol, methanol, ethylene glycol and other toxins in very high concentration, usually small molecules. 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