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).

Where:

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
Measured ETOH, plasma:	mg/dL

Osmolality Gap:

(unexplained after subtracting ethanol contribution)

97% of patients will have OG in the range +10 to -10.
An OG value greater than 15 is considered a critical value or cutoff.
The presence of low blood pH, elevated anion gap and greatly elevated OG is a medical emergency that requires prompt treatment.

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: see ref.³

ETOH, mg/dL
Ethylene Glycol, mg/dL
Methyl Alcohol, mg/dL
Isopropyl Alcohol, mg/dL

References

Khajuria, A., and Krahn, J: Osmolality revisited – deriving and validating the best formula for osmolality. Clin. Biochem. 38: 514-519, 2005.
Lynd LD et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning. BMC Emerg Med 8: 5, 2008. (3) Krasowski MD et al. A retrospective analysis of glycol and toxic alcohol ingestion: utility of anion and osmolal gaps. BMC Clin Pathol 12: 1, 2012.
Krasowski MD et al. A retrospective analysis of glycol and toxic alcohol ingestion: utility of anion and osmolal gaps. BMC Clin Pathol 12: 1, 2012.