Hyperosmolar therapy (mannitol or hypertonic saline) is commonly used in the treatment of neurocritical care paitents with elevated ICP.
When administering mannitol, guidelines recommend monitoring serum sodium and serum osmolarity. Though targets remain controversial, most strive for a serum sodium of 150-160 mEq/L and a serum osmolarity between 300 - 320 mOsm/L.
Unfortunately, serum osmolarity is a poor method to monitor mannitol therapy.
Instead of serum osmolarity, follow the osmolar gap. It is more representative of serum mannitol levels and clearance. If the osmolar gap falls to normal, the patient has cleared mannitol and may be redosed if clinically indicated.
References
Hinson HE, Stein D, Sheth KN. Hypertonic Saline and Mannitol in Critical Care Neurology. J Intensive Care Med 2013; 28:3-11.
