The osmolar gap is the difference between the measured osmolality and the calculated osmolarity.
The difference in units is attributed to the difference in the way that blood solutes are measured in the laboratory versus the way they are calculated. The laboratory value measures the freezing point depression, properly called osmolality while the calculated value is given in units of osmolarity. Even though these values are presented in different units, when there is a small amount of solute compared to total volume of solution, the absolute values of osmolality vs. osmolarity are very close. Often, this results in confusion as to which units are meant. For practical purposes, the units are considered interchangeable. The resulting "osmolar gap" can be thought of as either osmolar or osmolal, since both units have been used in its derivation. This is acceptable because, as stated above, the absolute values of these units will be very close under clinical circumstances, as because the value is meant only as an estimate to help in the differential diagnosis of the clinical issue at hand. So in medical terms Osmolality and Osmolarity are interchangeable.
Normal serum osmolality range: 280 - 290 mOsm/kg or 280-290 mOsm/L (reported by laboratory)
All but ~20 of the 290 mOsm in each liter of plasma is contributed by Na+ and it's accompanying anions: Cl- and HCO3-.
Osmolal Gap = Measured serum osmolality - Calculated serum osmolality
Calculated serum osmolality = 2 x Na + glucose/18 + BUN/2.8
If the measured osmolality exceeds the calculated value by >10
Causes of increased Osmolal Gap:
Decreased serum water
dehydration
hypernatremia,
DI
Uremia
Hyperglycemia
diuretic therapy
hyperproteinemia
hypertriglyceridemia
Presence of unmeasured osmoles
sorbitol, glycerol, mannitol, ethanol, isopropyl alcohol, acetone, ethyl ether, methanol, and ethylene glycol.