osmolal

Os molal is an adjective relating to osmolality, the measure of the concentration of osmotically active particles per kilogram of solvent. The common unit is osmoles per kilogram, often reported in milliosmoles per kilogram (mOsm/kg). Osmolality is distinct from osmolarity, which quantifies solute particles per liter of solution; in typical biological fluids at near-physiologic density, the two values are close, but osmolality is preferred because it is largely independent of temperature and volume changes.

In clinical practice, osmolality is used to assess fluid balance and solute load. Direct measurement is performed with an osmometer, most commonly by freezing-point depression; vapor-pressure osmometry is another method. A calculated osmolality can be estimated using common solutes: approximately 2 times the serum sodium plus glucose and BUN contributions, expressed as: calculated osmolality ≈ 2[Na+] + (glucose/18) + (BUN/2.8) when glucose and BUN are in mg/dL. Alcohols and other exogenous solutes raise the measured osmolality, creating an osmolal gap defined as measured osmolality minus calculated osmolality. A notable gap suggests presence of unmeasured osmoles such as methanol, ethylene glycol, isopropanol, or mannitol.

Normal plasma osmolality typically ranges about 275–295 mOsm/kg, with urine osmolality varying widely based on hydration and kidney function. Clinically, osmolality aids evaluation of hyponatremia and hypernatremia, guides fluid therapy, and helps identify toxic ingestions or excessive intake of osmotically active substances.