Estimate serum osmolality and compare it with a measured value when available.
Serum Osmolality
Form
Results
Serum osmolality
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Osmotic gap
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Table of Contents
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What is Serum Osmolality?
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How to Use This Calculator
- Enter or select Sodium, Blood urea nitrogen, Glucose, Serum alcohol concetration, Measured serum osmolality.
- Check the unit beside each measurement. For a percentage or rate, use the scale and time period stated in the form.
- Choose Calculate, then compare the output with the formula and example below. If you change a value or option, calculate again to update the result.
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Formula Used
Estimate = 2Na+glucose/18+BUN/2.8+ethanol/3.7, using mg/dL for the last three inputs.
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Variables Explained
| Input | What it means |
|---|---|
| Sodium | Sodium concentration, normally in mmol/L or mEq/L; follow the form’s unit label. |
| Blood urea nitrogen | Use the observed finding or laboratory value for this criterion, following the definition shown in the form. |
| Glucose | Glucose concentration in the stated unit; mg/dL and mmol/L differ by about a factor of 18. |
| Serum alcohol concetration | Use the observed finding or laboratory value for this criterion, following the definition shown in the form. |
| Measured serum osmolality | Use the observed finding or laboratory value for this criterion, following the definition shown in the form. |
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Worked Example
Sodium 140, glucose 90, BUN 14 and no ethanol give about 290 mOsm/kg.
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Why Use This Calculator?
Use the calculation to make the method and its inputs easier to check with a healthcare professional. Its meaning depends on the clinical setting and on whether the underlying method is appropriate for that person.
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Common Mistakes
- Osmolal-gap calculations depend on the selected equation; ethanol divisors differ between conventions.
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Frequently Asked Questions
Is this result a diagnosis or treatment instruction?
No. It describes a calculation or score from the supplied information. A qualified clinician must check the measurements, the method’s applicability and the wider clinical picture.
Can I enter the units from any lab report?
Use the units specified in the form and select a conversion only where the page provides one. Different substances need different conversion factors; a mismatch can produce a plausible but incorrect result.
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Sources & References
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