What Is Osmolality?


Quick Answer

Osmolality is an estimation of how much of a substance has been dissolved in another substance. In medical terms, it is a measure of the osmolar concentration of plasma, and is measured using an osmometer found in clinical laboratories. The osmolality of a fluid can also be calculated.

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In clinical laboratories, osmolality is measured using three types of osmometers: the vapor pressure osmometers, the freezing point osmometers and the membrane osmometers. Osmolality can also be calculated by adding the values of all the constituent solutes in a fluid. It is expressed as milliosmoles per kilogram of water, mOsmol/kg. The normal osmolality for extracellular fluids is 280 to 295 mOsmol/kg. The difference between the measured value and the calculated value is called the osmolar gap.

An osmolality test is used to determine the number of solutes present in urine, blood and stool. This test is crucial in evaluating the body's water balance, determining sodium levels and detecting the presence of toxins, such as ethylene glycol and methanol, in the body. The osmolality test is also crucial in monitoring the effectiveness of treatment for conditions affecting a person's osmolality and determining active drug therapies. Plasma, urine and stool osmolality is affected by water content changes.

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