Osmolality Gap Calculator

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The osmolality gap (OG) is an indication of unmeasured solute in the blood.
It is determined by the measured osmolality (MO) minus the calculated osmolality (CO).


Here's an easy way to calculate it.

Plasma sodium concentration: mmol/L
Plasma glucose: mg/dL
Plasma urea: mg/dL
Measured Osmolality, plasma: mOsm/kg H2O

 Osmolality Gap: mOsm/kg H2O

If the above Osmolality Gap is elevated, the amount of one of these candidate substances that may explain the elevation will be calculated by checking an option below:
ETOH, mg/dL see ref.3
Ethylene Glycol, mg/dL
Methyl Alcohol, mg/dL
Isopropyl Alcohol, mg/dL


  1. Glasser, L., et al. Serum Osmolality and its applicability to drug overdose. Am. J. Clin. Path. 60:695, 1973
  2. Jacobsen, D., et al. {series of three articles dealing with methanol and ethylene glycol overdose and use of anion gap and osmolality gap in diagnosis}. Acta. Med. Scand. 212:1-20, 1982.
  3. Khajuria, A., and Krahn, J. Osmolality revisited - deriving and validating the best formula for osmolality. Clin. Biochem. 38: 514-519, 2005.