Sodium Contribution
Na 140, Glucose 100, BUN 14
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Evidence-based calculations Used in clinical settings worldwide Regular monitoring recommended
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Normal Plasma Osmolality
Na 140, Glucose 100, BUN 14
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Hyperglycemic State
Elevated glucose causing hyperosmolality
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Hyponatremic Hypo-osmolar
Low sodium with decreased osmolality
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Hypernatremic Dehydration
Elevated sodium with high osmolality
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Uremic State
Elevated BUN in renal failure
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Toxic Alcohol Suspected
Elevated osmolar gap without ethanol
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Clinical Scenarios
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Normal Plasma Osmolality
Na 140, Glucose 100, BUN 14
Healthy adult with normal lab values
Hyperglycemic State
Elevated glucose causing hyperosmolality
HHS patient with severe hyperglycemia
Hyponatremic Hypo-osmolar
Low sodium with decreased osmolality
SIADH patient presenting with confusion
Hypernatremic Dehydration
Elevated sodium with high osmolality
Elderly patient with dehydration
Uremic State
Elevated BUN in renal failure
ESRD patient before dialysis
Toxic Alcohol Suspected
Elevated osmolar gap without ethanol
Altered patient with unknown ingestion
Electrolytes
Glucose
BUN / Urea
Measured Osmolality
Formula Selection
Optional Parameters
Plasma Osmolality Results
- Repeat osmolality to assess response to therapy
Osmolality Visualization
Step-by-Step Calculations
Osmolality Formulas Comparison
| Formula | Equation | Notes |
| Standard | 2×Na + Glu/18 + BUN/2.8 | Most widely used; simple |
| Dorwart-Chalmers | 1.86×Na + Glu/18 + BUN/2.8 + 9 | May be more accurate in some studies |
| Worthley | 2×(Na+K) + Glu/18 + BUN/2.8 | Includes potassium contribution |
| Bhagat | 1.86×(Na+5) + Glu/18 + BUN/2.8 | Alternative coefficient |
What is Plasma Osmolality?
Plasma osmolality represents the concentration of dissolved particles in plasma, expressed as milliosmoles per kilogram (mOsm/kg). It is primarily determined by sodium, glucose, and blood urea nitrogen (BUN), and is tightly regulated by the body between 275-295 mOsm/kg.
Measurement Methods
Measured by freezing point depression. Calculated from electrolytes and metabolites. Comparing both reveals unmeasured osmoles.
Clinical Importance
Affects water distribution between compartments. Influences thirst and ADH secretion. Abnormalities indicate serious metabolic disorders.
Osmolar Gap
Difference between measured and calculated osmolality. Normal is less than 10 mOsm/kg. Elevated gap suggests toxic alcohols or other unmeasured substances.
Key Osmolality Formulas
1. Standard Calculated Osmolality
2. Effective Osmolality (Tonicity)
Excludes BUN and ethanol - they cross cell membranes freely
3. Osmolar Gap
Normal: less than 10 mOsm/kg. Greater than 10 suggests unmeasured osmoles
Frequently Asked Questions
What is the difference between plasma and serum osmolality?
Clinically, plasma and serum osmolality are used interchangeably. Plasma is blood without cells but with clotting factors; serum lacks both. The osmolality values are virtually identical.
Why do different formulas exist?
Different formulas account for various factors and may perform better in specific populations. The standard formula (2×Na + Glu/18 + BUN/2.8) is most widely used due to simplicity.
Should I include potassium in the calculation?
Potassium contributes minimally to osmolality due to its low serum concentration (~4 mEq/L). Most standard formulas exclude it, but Worthley's formula includes it for completeness.
When is osmolar gap most useful?
Osmolar gap is most useful for screening for toxic alcohol ingestion (methanol, ethylene glycol). It's also elevated in ketoacidosis, lactic acidosis, and with mannitol administration.
Clinical Pearls
Sodium Dominates Osmolality
Sodium accounts for approximately 280 of the ~290 mOsm/kg in normal plasma. Changes in sodium directly impact osmolality and tonicity.
Tonicity Determines Cell Volume
Only tonicity (effective osmolality) causes water shifts and cell volume changes. BUN and ethanol cross membranes freely and don't affect cell volume.
Osmolar Gap Timing
In toxic alcohol ingestion, osmolar gap is elevated early then normalizes as the alcohol is metabolized, while anion gap rises. Timing is critical.
ADH Response
ADH release is triggered by just 1-2% increase in osmolality. The osmoreceptors in the hypothalamus are exquisitely sensitive to osmolality changes.
Pseudohyponatremia
Severe hyperlipidemia or hyperproteinemia can cause falsely low sodium measurements. In these cases, osmolality will be normal despite apparently low sodium.
Correction Rate Matters
Rapid correction of hyperosmolality risks cerebral edema. Rapid correction of hypo-osmolality risks osmotic demyelination syndrome. Target 8-12 mEq/L per 24 hours.
Related Calculators
For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
— WHO
— CDC
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