How to Calculate Osmolarity
What is Osmolarity?
Calculated serum osmolarity estimates plasma solute concentration using sodium, glucose, and BUN. Normal range: 275–295 mOsm/kg.
Formula
Serum osmolality = 2(Na) + (Glucose/18) + (BUN/2.8) + (Ethanol/4); Normal 275–295 mOsm/kg; High = hypernatremia or dehydration; Low = hyponatremia or overhydration
- Na
- Serum sodium (mEq/L (135–145 normal))
- Glucose
- Serum glucose (mg/dL)
- BUN
- Blood urea nitrogen (mg/dL)
- Osmolality
- Serum osmolality (mOsm/kg)
Step-by-Step Guide
- 1Osmolarity = 2×Na + Glucose(mg/dL)/18 + BUN(mg/dL)/2.8
- 2Normal: 275–295 mOsm/kg
- 3> 295: hyperosmolar (dehydration, DKA, hyperglycaemia)
- 4< 275: hypo-osmolar (SIADH, water overload)
Worked Examples
Input
Na 142, Glucose 108, BUN 14
Result
Osmolarity = 284 + 6 + 5 = 295 mOsm/kg (upper normal)
Frequently Asked Questions
What does osmolality measure?
Concentration of particles in serum. High = solute concentrated (dehydration, hypernatremia). Low = dilute (overhydration, hyponatremia). Critical for cell hydration status.
Why does high glucose increase osmolality?
Glucose acts as osmotic particle; draws water from cells (osmotic dehydration). Severe hyperglycemia (> 600 mg/dL) can cause hyperosmolar state (emergency).
What causes osmolality abnormalities?
High: dehydration, hypernatremia, renal failure (accumulates urea). Low: SIADH (inappropriate ADH), overhydration, liver disease (low osmolality + hyponatremia).
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