The components used in the plasma osmolality estimation formula are which of the following combinations?

The components used in the plasma osmolality estimation formula are which of the following combinations?

• A. Sodium, Glucose, and Urea nitrogen
• B. Sodium, Potassium, and Glucose
• C. Sodium only
• D. Glucose and Urea nitrogen

Answer: (A)

Plasma osmolality is set by the amount of osmotically active particles in the blood. The major determinant is sodium, because each sodium ion is accompanied by charge-balancing ions, effectively contributing about two osmoles for each sodium when estimating osmolality. The next important contributors are glucose and urea nitrogen, which add to the osmotic load. That’s why the estimation formula uses sodium, glucose, and urea nitrogen (often written as 2 × Na + glucose + BUN). The other options leave out one of these key contributors or replace sodium with another ion, which would misrepresent the main osmoles in plasma.

Plasma osmolality is set by the amount of osmotically active particles in the blood. The major determinant is sodium, because each sodium ion is accompanied by charge-balancing ions, effectively contributing about two osmoles for each sodium when estimating osmolality. The next important contributors are glucose and urea nitrogen, which add to the osmotic load. That’s why the estimation formula uses sodium, glucose, and urea nitrogen (often written as 2 × Na + glucose + BUN). The other options leave out one of these key contributors or replace sodium with another ion, which would misrepresent the main osmoles in plasma.