Osmolality and the Osmolar Gap

Osmolality and osmolar gap assist in the identification of osmotically active toxic alcohols. They are particularly useful when laboratory estimation of serum methanol and ethylene glycol levels are not readily available.

  • Osmolality refers to the number of osmotically active particles in a kilogram of solution (mOsmol/kg).  It is a measured value, usually determined by freezing point depression.
  • Osmolar gap is an artificial derived value, calculated by subtracting the calculated osmolality from the true osmolality measured by the laboratory.

Calculated Osmolality (mOsmol/kg) = 2 x [Sodium mmol/L] + [urea mmol/L] + [glucose mmol/L] + [ethanol mmol/L]

Osmolar Gap = Measured Osmolality – Calculated Osmolality

A normal osmolar gap is < 10. A small gap exists because the calculation does not take into account osmotic activity generated by chloride, potassium, sulphate, phosphate, calcium, magnesium, lactate, ammonia, serum proteins and lipids.


  • Koga Y, Purssell RA, Lynd LD. The irrationality of the present use of the osmole gap: applicable physical chemistry principles and recommendations to improve the validity of current practices. Toxicological Reviews 2004; 23(3):203-211.
  • Purssell RA, Lynd LD, Koga Y. The use of the osmole gap as a screening test for the presence of exogenous substances. Toxicological Reviews 2004; 23(3):189-202.

Toxicology Handbook

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