milliosmolality

Milliosmolality is a measure of the osmotic concentration of a solution, expressed as milliosmoles per kilogram of solvent (mOsm/kg). In physiology and clinical chemistry, it is commonly used to describe the osmolality of body fluids. Osmolality is per kilogram of solvent, whereas osmolarity is per liter of solution; milliosmolality specifically denotes the milliosmoles per kilogram.

Measurement is performed with an osmometer, typically using freezing point depression or vapor pressure methods. The result is reported in milliosmoles per kilogram (mOsm/kg) and reflects the total concentration of osmotically active particles in the fluid being tested, including electrolytes, glucose, urea, and other dissolved solutes.

Normal ranges vary by fluid. Plasma osmolality is generally about 275–295 mOsm/kg. Urine osmolality is highly variable, ranging from roughly 50 to over 1200 mOsm/kg depending on hydration status, renal function, and solute load. Clinically, the measured osmolality can be compared with a calculated osmolality (based on sodium, glucose, and urea) to derive an osmolar gap, which helps identify the presence of unmeasured solutes such as alcohols or toxins.

Interpretation centers on hydration and solute balance. Elevated milliosmolality can indicate dehydration, hypernatremia, hyperglycemia, or intake of osmotically active substances, while low milliosmolality can reflect overhydration or hyponatremia. The osmolar gap remains a useful tool for detecting clinically significant, unmeasured solutes in the bloodstream.