Serum Osmolality Calculator
Calculate estimated serum osmolality from sodium, glucose and BUN, and determine the osmolal gap when measured serum osmolality is available.
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Osmolal Gap = Measured Osmolality − Calculated Osmolality
What is Serum Osmolality?
Serum osmolality reflects the concentration of osmotically active particles present in serum. Sodium and its associated anions are major contributors, while glucose and urea also contribute to the calculated serum osmolality.
Calculated serum osmolality can be estimated from routinely available laboratory measurements of sodium, glucose and BUN. When measured osmolality is also available, the difference between measured and calculated osmolality is referred to as the osmolal gap.
Serum Osmolality Interpretation
| Calculated Osmolality | General Interpretation | Clinical Consideration |
|---|---|---|
| Below 275 mOsm/kg H₂O | Lower than typical reference range | May be seen with hypotonic states. Correlate with serum sodium, hydration status and clinical findings. |
| 275–295 mOsm/kg H₂O | Typical reference range | Generally compatible with normal serum osmolality when interpreted using the laboratory's reference interval. |
| Above 295 mOsm/kg H₂O | Higher than typical reference range | May occur with hypertonic states, including significant hypernatremia, hyperglycemia or increased effective osmoles. |
The reference interval shown is a commonly used general range. Individual laboratory reference intervals may differ.
Understanding the Osmolal Gap
The osmolal gap is calculated by subtracting calculated serum osmolality from measured serum osmolality.
A higher-than-expected osmolal gap may indicate the presence of unmeasured osmotically active substances. Clinical causes include certain toxic alcohol exposures and other exogenous or endogenous osmoles.
An elevated osmolal gap should not be interpreted in isolation. Clinical history, acid-base status, renal function, glucose, electrolytes and other laboratory findings should be considered.
Calculated vs Measured Serum Osmolality
| Parameter | Calculated Osmolality | Measured Osmolality |
|---|---|---|
| Basis | Derived from laboratory values | Direct laboratory measurement |
| Common inputs | Sodium, glucose and BUN | Osmometry |
| Main use | Estimation and comparison | Direct assessment of serum osmolality |
| Osmolal gap | Used as the calculated component | Used to calculate the measured-minus-calculated difference |
Clinical Applications of Serum Osmolality
Serum osmolality is useful when evaluating disorders of water balance, sodium abnormalities and changes in plasma tonicity.
It may be considered in the assessment of conditions associated with hyponatremia, hypernatremia, hyperglycemia and suspected abnormal osmoles.
The osmolal gap can provide an additional laboratory clue when the measured osmolality is substantially different from the calculated value.
Serum Osmolality Calculator - Frequently Asked Questions
What is the formula for calculated serum osmolality?
A commonly used formula is: 2 × sodium + glucose/18 + BUN/2.8, with glucose and BUN expressed in mg/dL and sodium in mmol/L.
What is the normal serum osmolality?
A commonly used reference range is approximately 275–295 mOsm/kg H₂O, although individual laboratory reference intervals may differ.
What is the osmolal gap?
The osmolal gap is the measured serum osmolality minus the calculated serum osmolality.
What does an increased osmolal gap indicate?
An increased gap may indicate additional unmeasured osmoles. Certain toxic alcohols and other substances can contribute to an increased osmolal gap.
Can calculated osmolality replace measured osmolality?
No. Calculated osmolality is an estimate. When direct measurement is clinically indicated, measured serum osmolality should be used.
Why is sodium the major component?
Sodium and its accompanying anions represent the major effective contributors to extracellular fluid osmolality.
Medical Disclaimer: This calculator is intended for educational and clinical reference purposes. Calculated serum osmolality is an estimate and should not replace direct laboratory measurement when measurement is clinically indicated. The calculated result and osmolal gap should be interpreted together with the patient's clinical condition, laboratory findings and the reference intervals used by the reporting laboratory.
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