Osmolal Gap is the difference between measured serum osmolality and calculated serum osmolality.
It is typically calculated as: OG = measured serum osmolality – (2 X serum sodium + serum glucose + serum urea)
* 2 X serum sodium + serum glucose + serum urea = the calculated serum osmolality and all measures are in mmol/L.
* OG = osmolal gap
In US customary units the calculated osmolarity is: ( 2 x sodium ) + glucose/18 + BUN/2.8.
A normal osmolal gap is < 12 mmol/L.
Causes of an elevated osmolal gap are numerous. Several causes are:
* ethanol intoxication
* methanol ingestion
* isopropanol ingestion
* ethylene glycol ingestion
Anion Gap Increases in Ethylene Glycol Poisoning (as the unmeasured ion is Anion)
Anion Gap Decreases in Lithium Toxicity (as the unmeasured ion is cation)
What will happen to Osmolal Gap in both Conditions
Suppose a person has BOTH Etylene Glycol Poisoning and Lithium Toxicity (Hypothetical – say a person with MDP attempted suicide), his anion gap may even be normal (depending on the degree of the toxicities)
But the Osmolal gap will be very high