Plasma osmolality is calculated with the formula:
(2 × Na + K) + (glucose/18) + (urea/2.8)
Lab values for glucose and urea are reported in conventional units (mg/dL) and need conversion to SI units (mmol/L). An alternate method of conversion is glucose × 0.0555 and urea × 0.357.
Normal = 280–300 mosm/kg.
The difference between the measured and calculated osmolality is the osmolal gap normally less than 10 mosm/kg. Elevated levels are seen in the presence of ethanol, methanol, isopropyl alcohol, and ethylene glycol.
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Osmolarity is a measure of the osmoles of solute per liter of solution. A capital letter M is used to designate units of mmol/L. Volume of solution changes with the amount of solute added and also with temperature and pressure changes.
Osmolality is a measure of the osmoles of solute per kilogram of solvent and is reported commercially using mOsm/kg. As the amount of solvent remains constant regardless of temperature and pressure changes, osmolality is preferred and is commonly used.
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