Reviewed and revised 24 November 2016
- 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
- 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
- ethylene glycol
- polyethylene glycol (IV lorazepam)
- propylene glycol (IV lorazepam, diazepam and phenytoin)
- glycine (TURP syndrome)
- maltose (IV IG – Intragram)
References and links
- 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)
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.