Serum Osmolality Calculator
Calculate estimated serum osmolality from sodium, glucose and blood urea nitrogen (BUN) and assess the result against the commonly used reference range.
Enter Serum Values
Enter values from the same blood sample where possible.
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Serum Osmolality Interpretation
| Calculated Osmolality | Interpretation | Clinical Consideration |
|---|---|---|
| < 275 mOsm/kg | Low serum osmolality | May be seen with hypotonic states, including some causes of hyponatremia. Interpret together with serum sodium, volume status and clinical findings. |
| 275–290 mOsm/kg | Common reference interval | Generally consistent with the commonly used normal serum/plasma osmolality range. |
| > 290 mOsm/kg | Increased serum osmolality | May occur with hypernatremia, significant hyperglycemia, increased urea or other osmotically active substances. |
Reference intervals may vary slightly between laboratories. Use the laboratory's validated reference interval when available.
What Affects Serum Osmolality?
| Parameter | Effect on Calculated Osmolality | Clinical Relevance |
|---|---|---|
| Sodium | Major determinant | Changes in sodium concentration have a substantial effect on calculated serum osmolality. |
| Glucose | Increases osmolality when elevated | Marked hyperglycemia can substantially increase calculated serum osmolality. |
| BUN | Contributes to calculated osmolality | Increased urea contributes to measured and calculated osmolality, although urea has limited effect on transmembrane water shifts because it crosses cell membranes relatively freely. |
| Unmeasured osmoles | May increase measured osmolality | Alcohols and other osmotically active substances can produce a difference between measured and calculated osmolality. |
Calculated Osmolality vs Measured Osmolality
Calculated serum osmolality can be compared with directly measured serum osmolality to assess the osmolar gap.
Osmolar Gap = Measured Osmolality − Calculated Osmolality
An increased osmolar gap may indicate the presence of additional osmotically active substances. Clinical interpretation should consider ethanol exposure, toxic alcohols, laboratory variation and the patient's overall clinical condition.
The osmolar gap should not be interpreted in isolation. The appropriate threshold depends on the laboratory method, calculation used and clinical setting.
What Is Serum Osmolality?
Serum osmolality reflects the concentration of dissolved osmotically active particles in serum. It is commonly used in the evaluation of disorders of sodium and water balance.
The major contributors to calculated serum osmolality are sodium, glucose and urea nitrogen. Direct measurement can be performed using an osmometer.
Calculated osmolality is particularly useful when evaluating disorders such as hyponatremia, hypernatremia, hyperglycemia and suspected osmolar-gap disorders.
When Is Serum Osmolality Useful?
Serum osmolality may be useful when investigating:
- Hyponatremia and suspected hypotonic states
- Hypernatremia
- Severe hyperglycemia
- Altered mental status when a metabolic cause is suspected
- Suspected ingestion of osmotically active substances
- Possible osmolar-gap disorders
Serum Osmolality Calculator - Frequently Asked Questions
What is the formula for calculated serum osmolality?
The commonly used formula in conventional units is: 2 × sodium + glucose/18 + BUN/2.8.
What is the normal serum osmolality range?
A commonly used reference interval is approximately 275–290 mOsm/kg, although the laboratory's validated reference interval should be used when available.
What causes low serum osmolality?
Low serum osmolality is commonly associated with hypotonic states and may occur in some patients with hyponatremia. Sodium, glucose, volume status and the clinical context should be reviewed together.
What causes high serum osmolality?
Increased serum osmolality may occur with hypernatremia, significant hyperglycemia, increased urea and the presence of other osmotically active substances.
What is the osmolar gap?
The osmolar gap is the difference between directly measured serum osmolality and calculated serum osmolality. An increased gap may suggest additional osmotically active substances.
Can calculated osmolality replace measured osmolality?
No. Calculated osmolality is an estimate. Direct measurement may be preferred when an osmolar gap is being evaluated or when the clinical situation requires accurate measurement.
Why is sodium multiplied by 2?
Sodium is accompanied by negatively charged counterions in extracellular fluid. The sodium term therefore represents the major contribution of sodium and its associated anions to serum osmolality.
Medical Disclaimer: This calculator is intended for educational and clinical reference purposes. Calculated serum osmolality is an estimate and should not be used as a standalone diagnostic test. Interpretation should consider serum sodium, glucose, BUN, measured osmolality when available, volume status, clinical history and other relevant laboratory findings. Reference intervals and clinical decision thresholds may vary according to laboratory method and clinical setting.
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