Osmolal Gap Calculator

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The osmolal gap measures the difference between measured serum osmolality (lab-tested) and calculated serum osmolality. It helps detect unmeasured osmoles like methanol, ethylene glycol, or ethanol in poisoning cases.

Formula:

Osmolal Gap = Measured Serum Osmolality − Calculated Serum Osmolality

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

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

🧮 Osmolal Gap Calculator (Conventional & SI Units)

📐 Formula:

Osmolal Gap = Measured Osmolality - Calculated Osmolality
Conventional: (2 × Na) + (Glucose / 18) + (BUN / 2.8) [+ Ethanol / 4.6 (optional)]
SI Units: (2 × Na) + Glucose + BUN [+ Ethanol × 1.25 (optional)]

📊 Normal Values:

Measured Osmolality: 275 - 295 mOsm/kg
Osmolal Gap: 0 - 10 mOsm/kg (higher values may indicate toxins)

🖊️ 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 Osmolal Gap: < 10 mOsm/kg.
Clinically Significant Gap: > 10 mOsm/kg (suggests unmeasured osmoles).

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 Considerations

Toxic Alcohols: Methanol, ethylene glycol, isopropanol.
Other Causes:
Ketoacidosis (elevated ketones).
Lactic acidosis.
Hyperlipidemia (pseudohyponatremia).
Limitations:
Ethanol may mask the gap if not included in the calculation.
Formulas assume normal serum proteins and lipids.

Excluded Factors

Does not account for volatile alcohols (e.g., acetone) unless specified.
Non-toxin causes (e.g., hypertriglyceridemia) may require additional testing.

Key Takeaway: The osmolal gap is a rapid screening tool for toxins but must be interpreted alongside clinical history and confirmatory tests (e.g., anion gap, blood gas).

🔬 Lab Pro Tip: Always verify units (mg/dL vs. mmol/L) and include ethanol in the calculation if intoxication is suspected!

Possible References Used

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