Laboratory Test Reference Guide

Nil

-10  to  +10 mmol/L

The formula for the osmolar gap is:

“Measured osmolality”  – “calculated osmolarity” = osmolar gap

The average difference for all healthy patients is close to zero with a spread of results from negative to positive. The  formula for the calculated osmo is : (Sodium  x 2) + urea + glucose + (ethanol x 1.25) For this calculation the reference interval (calculation specific) has been reported to be from -10 to +10. Ethanol, when present, contributes 25% more to the measured osmolality than expected. The osmolar gap is a quick but crude screening test for toxic alcohols. Acids such as lactate, beta hydroxybuterate and the metabolites of ethylene glycol and methanol do not increase the osmolar gap since these negatively charged molecules are associated with a sodium molecule and therefore accounted for by the calculation. Only uncharged molecules such as methanol, ethylene glycol, isopropanol and mannitol  increase the osmolar gap.

Patients with lactic acidocis or ketoacidosis may have a mildly elevated osmolar gap due to the release of  glycogen breakdown products  or presence of glycerol and acetone.

Limitations of an osmolar gap include:

Pseudohyponatraemia causes a falsely elevated osmolar gap. The blood gas sodium is not susceptible to pseudohyponatraemia. Toxic concentrations for ethylene glycol may be as low as 3 mmol/L and methanol 6 mmol/L. Since  the “normal”  osmolar gap will be negative in about 50% of the population, the osmolar gap may  be < 10 in some patients with toxic concentrations of these alcohols. These alcohols are rapidly metabolised, the period that the osmolar gap return to normal  within a few hours following ingestion of a toxic alcohol. In most severely poisoned patients the osmolar gap is > 25 shortly after ingestion. The combination of an increased anion gap and osmolar gap should raise suspicion of exposure to methanol or ethylene glycol. The laboratory can perform an urgent test for methanol and ethylene glycol.