Calculates estimated serum osmolality from sodium, glucose and urea in SI units.
Overview
Calculates estimated serum osmolality from sodium, glucose and urea in SI units.
Clinical Significance
Provides an estimate for comparison with measured serum osmolality. The difference is the osmolal gap.
When to Use
- Use when the required measurements are available, reliable and clinically relevant.
How It Is Calculated
- Calculated osmolality = 2 × sodium + glucose + urea.
Interpretation
- Provides an estimate for comparison with measured serum osmolality. The difference is the osmolal gap.
Worked Example
Enter valid values in the displayed units. MedicalC then applies: Calculated osmolality = 2 × sodium + glucose + urea.
Limitations
- The result does not establish a diagnosis or mandate treatment.
- Measurement error may be more important than arithmetic precision.
- Published thresholds may differ between populations and settings.
Clinical Pearls
- Document the source values and units with the final result.
- Review trends rather than isolated values where appropriate.
Common Mistakes
- Using the wrong unit.
- Combining values from different time points.
- Applying the result outside the intended measurement or clinical context.
Frequently Asked Questions
What does this calculator provide?
Calculates estimated serum osmolality from sodium, glucose and urea in SI units.
What formula is used?
Calculated osmolality = 2 × sodium + glucose + urea.
Can the result be used alone?
No. It must be interpreted with the full clinical assessment and appropriate investigations.
What is the main limitation?
The result depends on accurate source measurements, correct units and appropriate clinical applicability.
References
- Purssell RA et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001. — Primary, supporting or authoritative reference for the implemented relationship.
Reviewed by: MedicalC Clinical Editorial Team
Last reviewed: July 2026