Osmolar Gap

The Osmolar Gap is another important diagnostic tool that can be used in differentiating the causes of elevated anion gap metabolic acidosis. The major osmotic particles in plasma are Na+ , Cl- , HCO3-, urea and glucose and as such, plasma osmolarity can be estimated as follows:
                  Plasma osmolarity = 2(Na) + glucose/18 + BUN/2.8

Note that because Cl- and HCO3- are always bound to Na, their contributions to osmolarity are estimated by doubling the Na concentration. Plasma osmolality (Posm) can also be measured directly by freezing point depression. The osmolar gap is the difference between the calculated serum osmolarity and the measured serum osmolarity.

Osmolar Gap = Measured Posm – Calculated Posm

The normal osmolar gap is  10-15 mmol/L H20 .The osmolar gap is increased in the presence of low molecular weight substances that are not included in the formula for calculating plasma osmolarity. Common substances that increase the osmolar gap are ethanol, ethylene glycol, methanol, acetone, isopropyl ethanol and propylene glycol.

lightbulbIn a patient suspected of poisoning, a high osmolar gap (particularly if ≥ 25) with an otherwise unexplained high anion gap metabolic acidosis is suggestive of either methanol or ethylene glycol intoxication.

One must correlate an elevated osmolar gap with other clinical findings because it is a relatively nonspecific finding that is also commonly seen in alcoholic and diabetic ketoacidosis, lactic acidosis and in chronic renal failure. Elevation in the osmolar gap in these disease states is thought to be due in part to elevations of endogenous glycerol, acetone, acetone metabolites, and in the case of renal failure, retention of unidentified small solutes.


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