How is plasma osmolality calculated?

The calculation of plasma osmolality is based on the concentration of solutes in the plasma that contribute significantly to its osmotic pressure. The formula for calculating plasma osmolality includes the major contributors to plasma osmolality, which are sodium, glucose, and urea. The correct formula, which is used clinically to estimate plasma osmolality, is expressed as:

Posm = 2(Na) + (glucose/18) + (BUN/2.8)

In this equation:

• The term 2(Na) accounts for the effect of sodium ions, which are predominant in determining osmotic balance.

• Glucose is included and divided by 18 because glucose contributes to osmolality, and the division converts the concentration from mg/dL to mmol/L.

• BUN (blood urea nitrogen) is also included, divided by 2.8 to adjust for its contribution to osmotic pressure since this conversion similarly transforms the measurement into mmoles/L.

This comprehensive formula accurately reflects the primary solutes contributing to plasma osmolality, providing a reliable estimation that can be critical in various clinical settings, especially when assessing fluid and electrolyte balance or evaluating conditions such as hypernatremia or hyponatremia