Which conditions can cause high plasma osmolality, contributing to hypernatremia?

• A. Hyperglycemia
• B. Mannitol infusion
• C. Both hyperglycemia and mannitol infusion
• D. Neither

Answer: (C)

Plasma osmolality rises with the amount of osmotically active particles in the blood. The main contributors are sodium (plus its accompanying anions), glucose, and urea, and any other osmoles present. When a patient has hyperglycemia, the elevated glucose adds directly to the osmotic load of the plasma, increasing osmolality. This osmotic effect pulls water from cells into the extracellular space, which can concentrate sodium and contribute to hypernatremia.

Mannitol works similarly but as an exogenous osmole. As it is not metabolized quickly and stays in the extracellular space, it raises plasma osmolality by increasing the number of particles in the blood. If the body can’t excrete it rapidly enough, osmolality remains high and can drive hypernatremia.

Since both elevated glucose and mannitol infusion increase plasma osmolality, either situation can contribute to hypernatremia.