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Osmolal Gap Calculator
The Osmolal Gap is the difference between the Measured Osmolality (determined by the laboratory using freezing point depression) and the Calculated Osmolality (estimated from major solutes).
An elevated osmolal gap suggests the presence of unmeasured osmotically active substances in the blood, such as toxic alcohols (methanol, ethylene glycol) or metabolic byproducts. It is a critical screening tool in toxicology.
Measured / Solutes
InputsmOsm/kg H₂O (Lab Result)
mEq/L
mg/dL
mg/dL
Results
Normal Gap: ≤ 10 mOsm/kg
Clinical Context
Formulas (US Units):
1. Calculated Osm = (2 × Na) + (Glucose / 18) + (BUN / 2.8)
2. Osmolal Gap = Measured Osm − Calculated Osm
Interpretation:
• Normal Gap (< 10): Indicates the measured solutes (Na, Glu, BUN) account for nearly all osmolality.
• Elevated Gap (> 10): Indicates unmeasured solutes. Common causes:
– Toxic Alcohols: Methanol, Ethylene Glycol, Isopropanol.
– Metabolic: Ketoacidosis (acetone), Lactic Acidosis.
– Medications: Mannitol, Propylene Glycol (IV vehicle).
– Other: Ethanol (If patient is intoxicated, correct the gap using ethanol level).
Clinical References
How to Use This Calculator
Follow these steps to calculate the Osmolal Gap.
Input Measured Osmolality
Enter the value from the lab report (usually 275-295 mOsm/kg).
Input Electrolytes
Enter Serum Sodium, Glucose, and BUN from the metabolic panel (BMP/CMP).
Interpret Result
The Calculated Osmolality and the Gap will display. A gap >10 warrants investigation for toxins.
Frequently Asked Questions
This converts mg/dL to mmol/L. The molecular weight of glucose is 180 g/mol.
Ethanol increases the measured osmolality. If the patient has consumed alcohol, the gap will be high. You can account for it: Gap – (Ethanol level / 4.6).