Measuring Serum Osmolality via VBG
You cannot directly measure serum osmolality from a VBG sample, but you can calculate it using the validated osmolarity equation: osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all in mmol/L). 1
Understanding the Distinction
Serum osmolality is directly measured using freezing point depression methodology in a laboratory osmometer 1. A VBG analyzer does not have this capability—it measures electrolytes, glucose, and other parameters, but not osmolality itself.
The Calculation Method from VBG Values
When direct osmolality measurement is unavailable, use the following approach:
The Validated Equation
osmolarity = 1.86 × (Na+ + K+) + 1.15 × glucose + urea + 14 (all measured in mmol/L) 1
This equation has been specifically validated in older adults across diverse clinical settings (community, residential care, hospital) and in patients with diabetes, poor renal function, and varying degrees of dehydration 1. It has also proven useful in younger adults 1.
Clinical Thresholds
- >295 mOsm/L: Screen positive for low-intake dehydration (sensitivity 85%, specificity 59%) 1
- >300 mOsm/L: Diagnostic threshold for dehydration when using directly measured osmolality 1
Blood Sampling Considerations for VBG
Preferred Sampling Sites
Use arterial or venous whole blood sampling from a catheter rather than finger-stick capillary sampling for critically ill patients, especially those on vasopressors, in shock, or with severe peripheral edema 2.
- Arterial samples provide values most similar to laboratory plasma measurements 2
- Venous samples are generally acceptable if you avoid contamination from IV fluid infusing through multilumen catheters 2
- Capillary blood from fingertick can show unpredictable variability in hypoperfused states 2
Important Caveats
The calculated osmolarity is an estimate, not a true measurement 1. The terminology matters:
- Osmolality = directly measured using freezing point depression
- Osmolarity = calculated estimate from an equation
Most published osmolarity equations are not diagnostically accurate in older adults 1. The equation above is the exception, having been validated across multiple European cohorts 1.
When Calculation Results Are Elevated
If your calculated osmolarity is >295 mOsm/L:
- Check that glucose and urea are within normal range 1. Elevated glucose or urea will falsely elevate osmolality without indicating true dehydration
- Normalize glucose and urea first if elevated 1
- In low-intake dehydration, it's common that no single component (sodium, potassium, urea, glucose) is elevated out of normal range, but general fluid concentration causes small rises within normal range across all components 1
- Obtain a directly measured serum osmolality a few days later to confirm and guide more intensive intervention 1
Monitoring During Treatment
For patients with diabetic ketoacidosis or hyperosmolar states, venous pH can be followed instead of repeated arterial blood gases to monitor resolution of acidosis 3. Generally, venous pH runs approximately 0.03 units lower than arterial pH 3.
Blood should be drawn every 2-4 hours for serum electrolytes, glucose, and calculated osmolality during acute management 3.