Serum osmolality measures the concentration of dissolved particles (solutes) per kilogram of blood plasma. It helps assess:
- Toxic alcohol ingestion (methanol, ethylene glycol)
- Hydration status (dehydration vs. overhydration)
- Electrolyte imbalances (hyponatremia, hypernatremia)
Formulas & Units:
Calculated Serum Osmolality
- Using mg/dL (Conventional Units):
Osmolality (mOsm/kg) =
(2 × Na⁺)
+
Glucose (mg/dL)
18
+
BUN (mg/dL)
2.8
+
[
Ethanol (mg/dL)
4.6
]
*Ethanol term optional (for osmolal gap calculation)
- Using mmol/L (SI Units):
Osmolality (mOsm/kg) =
(2 × Na⁺)
+ Glucose (mmol/L)
+ BUN (mmol/L)
[+ Ethanol (mmol/L) × 1.25]
*Ethanol term optional (multiply by 1.25 for osmolal gap)
*Conversion: 1 mmol/L ethanol = 4.6 mg/dL
*Conversion: 1 mmol/L ethanol = 4.6 mg/dL
Note: Ethanol (if present) can be added to the formula:
Ethanol Contribution =
Ethanol (mg/dL)
4.6
or
Ethanol (mmol/L)
×
1.25
*Conversion: 1 mmol/L ethanol = 4.6 mg/dL | Used to calculate osmolal gap
🧮 Serum Osmolality Calculator (Conventional & SI Units)
📐 Formula:
- Conventional: (2 × Na) + (Glucose / 18) + (BUN / 2.8) [+ Ethanol / 4.6 (optional)]
- SI Units: (2 × Na) + Glucose + BUN [+ Ethanol × 1.25 (optional)]
📊 Normal Value:
- Serum Osmolality: 275 - 295 mOsm/kg
🖊️ Enter the Values:
Purpose
- Screen for toxic alcohol ingestion (methanol, ethylene glycol).
- Identify osmotically active substances (e.g., mannitol, acetone).
- Evaluate electrolyte imbalances or lab errors.
Normal Value:
- Normal serum osmolality: 275–295 mOsm/kg
- Clinically significant deviations:
- < 275 mOsm/kg → Overhydration, SIADH (Syndrome of Inappropriate ADH)
- > 295 mOsm/kg → Dehydration, hyperglycemia, toxic alcohol ingestion
Example Calculation (mg/dL)
- Lab Values:
- Na⁺ = 140 mEq/L
- Glucose = 180 mg/dL
- BUN = 28 mg/dL
- Measured Osmolality = 320 mOsm/kg
- Calculated Osmolality:
Osmolality (mOsm/kg) = 2×Na⁺ +
Glucose (mg/dL)
18
+
BUN (mg/dL)
2.8
Osmolality (mOsm/kg) = 2× 140 +
180
18
+
28
2.8
Osmolality (mOsm/kg) = 280 + 10 + 10 = 300 mOsm/kg
- Osmolal Gap:
Osmolal Gap = Measured Serum Osmolality − Calculated Serum Osmolality
Osmolal Gap = 320 − 300 = 20mOsm/kg
How to Use the Calculator
- Input Values:
- Enter serum Na⁺, glucose, BUN, and measured osmolality.
- Select units (mg/dL or mmol/L).
- Calculate: Automatically computes the osmolal gap.
- Interpretation:
- Gap > 10: Investigate for toxins, ketoacidosis, or lab error.
- Gap < 10: Likely no significant unmeasured osmoles.
Clinical Applications
- Hyponatremia Workup (SIADH vs. pseudohyponatremia)
- Diabetic Ketoacidosis (DKA) Monitoring
- Toxic Alcohol Poisoning Screening
- Dehydration Assessment
Limitations
- Lipemia or hyperproteinemia may falsely alter lab readings.
- Does not account for all osmotically active substances (e.g., mannitol, glycine).
- Ethanol can mask osmolal gap if not included in calculations.
🔬 Lab Pro Tip: Always verify units (mg/dL vs. mmol/L) and include ethanol in the calculation if intoxication is suspected!
