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Calculated Plasma Osmolality Model

Estimate plasma osmolality from sodium, glucose and blood urea nitrogen.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Health results are educational estimates, not a diagnosis or treatment recommendation. A qualified clinician can interpret measurements in the context of your health.

Formula and method

Sodium 140 mmol/L, glucose 90 mg/dL and BUN 14 mg/dL give calculated osmolality 290 mOsm/kg.

Calculated osmolality = 2 × sodium + glucose/18 + BUN/2.8.

Worked example

Enter these known values and leave the other values blank.

Sodium
140 mEq/L
Glucose
90 mg/dL
Blood urea nitrogen
14 mg/dL
Plasma osmolality
290 mOsm/kg H₂O

Assumptions and limitations

  • Uses sodium in mmol/L and glucose and blood urea nitrogen in mg/dL. BUN is not total urea. This is calculated rather than measured osmolality and omits ethanol and other unmeasured solutes. Retained input scope: sodium above 99 and below 200, glucose above 49 and below 150, BUN above 1 and below 30.
  • Use the displayed units and measurements from the same context. Editable fields support arithmetic inverses when enough independent values are supplied. Clear a preset value before solving it as an unknown.
  • This tool performs the stated arithmetic model. It does not establish a diagnosis, prescribe treatment or replace laboratory and clinical assessment.

Common questions

What does this calculation represent?

Uses sodium in mmol/L and glucose and blood urea nitrogen in mg/dL. BUN is not total urea. This is calculated rather than measured osmolality and omits ethanol and other unmeasured solutes. Retained input scope: sodium above 99 and below 200, glucose above 49 and below 150, BUN above 1 and below 30.

Can I solve for a different field?

Clear the unknown field and enter enough independent values in the remaining editable controls. A relationship cannot recover a unique value when a dividing quantity is zero.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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