Calculated Serum Osmolarity

\[Osm=2Na+\frac{Glu}{18}+\frac{BUN}{2.8}\]

Variables

Osmcalculated serum osmolarity (mOsm/L)

Na[Na⁺]serum sodium concentration (mEq/L)

Gluserum glucose concentration (mg/dL)

BUNblood urea nitrogen concentration (mg/dL)

Description

What is this formula?

The Calculated Serum Osmolarity formula estimates the concentration of osmotically active particles in blood plasma using routinely measured laboratory values.

When to use it

Use this formula when evaluating fluid balance, electrolyte disorders, hyperglycemia, kidney disease, dehydration, or suspected osmolar disturbances.

Example

Data:

Na = 140 mEq/L

Glu = 100 mg/dL

BUN = 14 mg/dL

Formula:

Osm = 2Na + Glu/18 + BUN/2.8

Substitution:

Osm = (2 × 140) + (100/18) + (14/2.8)

Osm = 280 + 5.56 + 5

Result:

Osm = 290.6 mOsm/L

The calculated serum osmolarity is approximately 291 mOsm/L.

Applications

- Electrolyte disorder evaluation

- Dehydration assessment

- Hyperglycemic emergencies

- Nephrology

- Critical care medicine

Note

This formula is an approximation based on the major contributors to serum osmolarity. It does not account for alcohols, mannitol, ethylene glycol, methanol, or other osmotically active substances. Differences between measured and calculated osmolarity may indicate the presence of unmeasured osmoles.