mOsmkg

mOsm/kg, or milliosmoles per kilogram, is a unit of osmotic concentration used to express the number of osmotically active solute particles in a kilogram of solvent. It is commonly written as mOsm/kg or mOsmol/kg. In physiology and medicine, osmolality refers to the amount of solute per kilogram of solvent, whereas osmolarity denotes solute per liter of solution; in dilute aqueous solutions the two values are similar, but osmolality is preferred because it is not affected by temperature or volume changes.

Osmolality is determined by the total number of dissolved particles, including ions, glucose, and urea. In clinical practice, the major contributors to serum osmolality are sodium, glucose, and blood urea nitrogen (BUN). A commonly used calculation for estimated serum osmolality is 2[Na+] + [glucose]/18 + [BUN]/2.8, with results in mOsm/kg. This approximation excludes less abundant solutes and assumes typical dissociation of salts. Measured osmolality, obtained with an osmometer, reflects all osmotically active particles and may exceed calculated osmolality if substances such as ethanol, methanol, or mannitol are present, creating an osmolar gap.

Normal plasma osmolality typically ranges from about 275 to 295 mOsm/kg. Clinically, osmolality helps assess hydration status and electrolyte disturbances and guides fluid therapy. It is also used to evaluate suspected hyperosmolar states or poisoning; a large osmolar gap can indicate the presence of unmeasured solutes.

In summary, mOsm/kg describes the concentration of solute particles per kilogram of solvent, with osmolality measurements and calculations providing essential tools in clinical assessment of fluid balance and solute load.