How the Serum Osmolality Calculator works
Osmolality describes the concentration of dissolved particles in blood. Sodium (with its accompanying anions), glucose and urea account for most of it, so osmolality can be estimated from these three routine results.
Calculated osmolality is often compared with osmolality measured by an osmometer. A large difference (the osmolal gap) can point clinicians toward unmeasured substances.
Required inputs
- Sodium in mEq/L (= mmol/L)
- Glucose in mg/dL or mmol/L
- BUN in mg/dL, or urea in mmol/L
Formula used
In SI units this is equivalent to (2 × Na⁺) + glucose (mmol/L) + urea (mmol/L). The calculator converts SI values to mg/dL first (glucose × 18, urea × 2.8).
Worked example
Na⁺ 140 mEq/L, glucose 90 mg/dL, BUN 14 mg/dL:
- (2 × 140) + (90 ÷ 18) + (14 ÷ 2.8) = 280 + 5 + 5 = 290 mOsm/kg
Interpreting the result
A commonly cited reference interval for serum osmolality is about 275–295 mOsm/kg; laboratory intervals vary. Values outside this range can be associated with changes in water balance, sodium, glucose or urea, and should be interpreted in clinical context.
Limitations
- The output is calculated osmolality. Measured osmolality may differ, for example in the presence of alcohols (ethanol, methanol, ethylene glycol), mannitol or other osmotically active substances.
- Several published equations exist; they give slightly different results.
- Very high lipids or proteins can cause falsely low measured sodium on some analyzers (pseudohyponatremia).
- Use values from the same blood sample.