18 - Osmolality measurement

The osmolality of a solution can be measured using an osmometer. The most commonly used instrument in modern laboratories is a freezing point depression osmometer. This instrument measures the change in freezing point that occurs in a solution with increasing osmolality. Osmolality can be measured in samples of serum (gold top tube) or heparin plasma (lime top tube).


Plasma osmolality can also be calculated from the measured components. While there are many equations, a simple one is as follows:
Osmolality (calc) = 2 x Na + Glucose + urea (all measurements in mmol/L). 
The doubling of sodium accounts for the negative ions associated with sodium and the exclusion of potassium approximately allows for the incomplete dissociation of sodium chloride. The difference between the measured and calculated plasma osmolality is known as the osmolar gap and normally is between 0 and 10 mmol/kg.