Osmolar Gap

Reviewed and revised 24 November 2016

OVERVIEW

• an osmole is the amount of a substance that yields the number of particles that would depress the freezing point of the solvent by 1.86K
• Avogadro’s number of particles in an ideal solution
• osmolality is measured in laboratory by osmometers
• the units of osmolality are mOsm/kg of solute

Importance of the type of osmometer

• only osmometers using freezing point depression method can detect volatile alcohols which can abnormally increase the osmolar gap
• vapour pressure osmometers cannot do this

CALCULATION

• Calculated osmolarity = (2 x [Na+]) + [glucose] + [urea])
• Osmolar gap = Osmolality (measured) – Osmolarity (calculated)
• normal = < 10
• note that is a pragmatic clinical aid – the units are different (osmolality =mOsm/kg and osmolarity = mOsm/L) so it doesn’t make mathematical sense!

MEANING OF A HIGH OSMOLAR GAP

• presence of an abnormal solute present in significant amounts
• must have: a low molecular weight and be uncharged -> can elevate the osmolar gap
• if the ethanol levels are measured they can be added to the calculated osmolarity to exclude the presence of an additional contributor to the osmolar gap. [NB: To convert ethanol levels in mg/dl to mmol/l divide by 4.6]

CAUSES OF HIGH OSMOLAR GAP

if elevated consider presence of other osmotically active particles

• mannitol
• methanol
• ethylene glycol
• sorbitol
• polyethylene glycol (IV lorazepam)
• propylene glycol (IV lorazepam, diazepam and phenytoin)
• glycine (TURP syndrome)
• maltose (IV IG – Intragram)

References and links

Journal articles

• Gennari FJ. Current concepts. Serum osmolality. Uses and limitations. N Engl J Med. 1984 Jan 12;310(2):102-5. PMID: 6361557.

FOAM and web resources

• EMNerd — You don’t understand the Osm gap (2015)