What does osmolality mean?
Osmolality is the concentration of osmotically active particles in a solution, measured in osmoles per kilogram of solvent. The term is especially common in clinical descriptions of blood plasma, serum, and urine, where it helps indicate how strongly water would tend to move into or out of a fluid. A person's plasma osmolality is closely related to the balance between water and dissolved substances such as sodium salts, glucose, and urea. Abnormally high values can be associated with dehydration or increased concentration of certain solutes, while abnormally low values may accompany excessive dilution. Although osmology and osmolality are sometimes used informally in discussion of bodily fluids, the precise measurement is important in diagnosing fluid and electrolyte disorders. Osmolality should not be confused with osmolarity: osmolality uses the mass of the solvent as its denominator, while osmolarity uses the volume of the entire solution.
nounOsmolality is the concentration of osmotically active particles in a solution, expressed as osmoles per kilogram of solvent. It is especially important as a measure of the solute concentration of blood plasma, serum, or urine.
- The concentration of osmotically active particles in a solution, expressed as osmoles per kilogram of solvent.
- The osmotic concentration of a biological fluid, especially blood plasma, serum, or urine, used to assess the balance between water and dissolved solutes.
"The patient's plasma osmolality rose sharply because of severe water loss."
"The laboratory reported a plasma osmolality of 295 mOsm/kg."
"A urine osmolality test helped determine whether the kidneys were concentrating the urine appropriately."
The plural is used when referring to measurements of osmolality from different samples, solutions, or subjects.
"The researchers compared the osmolalities of plasma and urine samples."
Plasma osmolality helps clinicians determine whether a patient's body fluids are too concentrated or too dilute, making it a key tool in diagnosing dehydration and electrolyte disturbances.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of osmolality
Osmolality was formed within the scientific vocabulary derived from Greek osmē, meaning a pushing or impulse, which also underlies osmosis and osmotic. The suffix -ality forms a noun of measurement or quality, parallel to terms such as modality and nationality. Its name reflects the word's relationship to osmosis, the movement of solvent across a semipermeable membrane toward a solution of greater particle concentration.
How osmolality is actually used
Osmolality is a technical term used chiefly in medicine, physiology, and laboratory science. Because it is calculated per unit mass of solvent, it differs slightly from osmolarity, which is calculated per unit volume of solution.
Easily confused with osmolality
Osmolality measures the concentration of dissolved particles per kilogram of solvent, whereas osmolarity measures it per liter of solution.