15/07/2025
✅ How to Calculate Serum Osmolality (Osmolarity):
The most commonly used formula for calculating serum osmolality is:
Serum Osmolality (mOsm/kg) =
(2 × Na⁺) + (Glucose ÷ 18) + (BUN ÷ 2.8)
---
📌 Units Used:
Na⁺ in mEq/L
Glucose in mg/dL
BUN in mg/dL
---
🔍 Example Calculation:
If:
Na⁺ = 140 mEq/L
Glucose = 90 mg/dL
BUN = 14 mg/dL
Then:
Osmolality = (2 × 140) + (90 ÷ 18) + (14 ÷ 2.8)
Osmolality = 280 + 5 + 5
= 290 mOsm/kg
---
✅ Normal Range:
275–295 mOsm/kg
---
💡 Notes:
This formula estimates effective osmolality (excluding alcohols, toxins, or mannitol).
If there is a large gap between measured and calculated osmolality (>10 mOsm/kg), suspect:
Toxins like methanol, ethylene glycol, isopropanol
Mannitol, sorbitol, or other osmotically active substances
_____________________________________
The difference between osmolality and osmolarity:
✅ 1. Definition
Osmolality:
➤ Number of osmoles of solute per kilogram of solvent (water)
➤ Units: mOsm/kg H₂O
Osmolarity:
➤ Number of osmoles of solute per liter of solution
➤ Units: mOsm/L
---
✅ 2. Key Practical Difference
Osmolality is more accurate in clinical settings because it’s independent of temperature and pressure.
Osmolarity can change with volume shifts due to temperature or solute effects on volume.
---
✅ 3. Clinical Use
Serum osmolality is the standard measurement in medicine (used in hyponatremia, DI, SIADH).
Osmolarity is used more in laboratory and IV fluid calculations.
---
✅ 4. Analogy
Think of:
Osmolality like measuring per weight (kg) — stable
Osmolarity like measuring per volume (L) — variable
---
✅ In Practice
> The difference is minor in dilute solutions like plasma, so they're often used interchangeably — but technically, osmolality is preferred clinically.
