Osmolality Gap Calculator

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

Where:
  CO = 2 X [Na]+[glucose (mg/dl)] / 20 + [urea (mg/dl) ] / 3 - 2

         Osmolality Gap = MO - 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

60% of patients will have OG in the range +3 to -3.
An OG value greater than 14 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:

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

References

Glasser, L., et al. Serum Osmolality and its applicability to drug overdose. Am. J. Clin. Path. 60:695, 1973
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.

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

Osmolality Gap:	mOsm/kg H2O