We explored the Five W’s of Osmolality Testing in the most recent blog. These 5 W’s are often followed by a sixth interrogative question, “How?” Now, THIS is the nerdy fun part! Let’s go!

The How of Osmolality Testing


Osmolality testing uses state of the art pieces of laboratory instruments known as osmometers. There are two main types, vapor pressure osmometers, and freezing point osmometers.  

VAPOR PRESSURE osmometry works by taking advantage of the decrease in vapor pressure that occurs when solutes are added to the pure solvent. The higher the concentration of particles in a solution, the higher the osmolality, and the lower the vapor pressure when heated. The VPO (vapor pressure osmometer) uses thermistors, a thermostated chamber, and electrical circuit, to measure that difference and calculate the osmolality of the sample.  https://apps.dtic.mil/dtic/tr/fulltext/u2/487755.pdf

Vapor pressure osmometry has been around for several decades, continues to increase in prevalence, and has been found quite suitable for many fluids in biology and medicine where water is the solvent. 

FREEZING POINT osmometry determines the osmotic strength of a solution by utilizing freezing point depression. A sample is introduced to a cooling chamber where it is cooled to a temperature below its expected freezing point. The sample is stirred with a wire, which causes the sample to become partially crystallized. The heat generated from this process brings the sample to a temperature that is equal to its freezing point. A standard freezing curve is generated, and VOILA, the osmolality of the sample is calculated.

The osmolality by freezing point test is an important diagnostic method often preferred in chemistry, pharmaceutical and quality control labs.

https://www.khanacademy.org/test-prep/mcat/physical-sciences-practice/physical-sciences-practice-tut/e/-using-a-freezing-point-depression-osmometer-to-measure-serum-osmolality 

The How of Osmolality Testing

The Osmolality Lab employs both the vapor pressure and freezing point methods of osmolality determination depending on the sample being analyzed, and of course, the preference of our clients when both methods 

are appropriate. The Osmolality Lab employs the USP <785> assay method, but can also use other alternative methods that have been developed and validated for the client’s product. Shoot off a quick inquiry to the scientists at The Osmolality Lab to find out which method is best for you! 

https://osmolab.com/contact-us/

Scott Dalton
Operations Director
The Osmolality Lab