How is osmolality calculated?

Osmolality is an important measurement in clinical settings, providing insights into a patient's hydration status and electrolyte balance. It is calculated using the formula that includes sodium (Na), blood urea nitrogen (BUN), and glucose, because these components significantly influence plasma osmolality.

The correct formula for determining osmolality is 2 * Na + 1/3 * BUN + 1/18 * Glucose. Each of these factors contributes to the overall osmotic pressure in the blood:

1. Sodium (Na) is the primary determinant of osmolality in the extracellular fluid. It accounts for the majority of osmotic activity and is multiplied by 2 in the formula because it represents two moles of osmotic activity per mole (sodium ions can dissociate).

2. BUN (Blood Urea Nitrogen) contributes to osmolality but is often considered at a lower fractional weight because urea is not as effective an osmole as sodium. When calculating osmolality, BUN is divided by three to reflect its lesser contribution.

3. Glucose also contributes to osmolality and is included in the calculation. However, it is divided by 18, as its contribution is generally