To calculate the osmolar gap, you subtract the calculated osmolarity from the measured osmolality: Osmolar Gap = Measured Osmolality – Calculated Osmolarity. 

Here’s a more detailed explanation:

• Measured Osmolality:This is the actual concentration of particles (osmoles) in the blood, measured in a lab.Â
• Calculated Osmolarity:This is an estimate of the osmolality based on the concentrations of common solutes like sodium, glucose, and urea.
  • The formula for calculating osmolarity is: 2 x [Na+] + [Glucose/18] + [BUN/2.8].
    • [Na+] is the sodium concentration in mmol/L.Â
    • [Glucose] is the glucose concentration in mg/dL.Â
    • [BUN] is the blood urea nitrogen concentration in mg/dL.Â
    • The numbers 18 and 2.8 are used to convert glucose and BUN from mg/dL to mmol/L.Â
• Osmolar Gap:This represents the difference between the measured osmolality and the calculated osmolarity, indicating the presence of unmeasured solutes in the blood.
  • A normal osmolar gap is typically less than 10 mOsm/kg.Â
  • An elevated osmolar gap can indicate the presence of substances like ethanol, methanol, ethylene glycol, or other toxic alcohols.Â