Plasma osmolality is the osmolality of human blood. Normal osmolality in plasma is about 280 - 303 milli-osmoles per kilogram. This is contributed to mainly by sodium, chloride, potassium, urea, and glucose, and additionally by other ions and substances in the blood.

Plasma osmolality can be approximated with the following formula:

US units (sodium as mEq/l, BUN and glucose as mg/l) Plasma osmolality (mOsm/l) = 2(+ ^*/2.8)" target="_blank" >+ ([Glucose/18)

SI units (all variables in mmol/l): Plasma osmolality (mOsm/l) = 2(+ ^*" target="_blank" >+ [Glucose

As cell membranes in general are freely permeable to water, the osmolality of the extracellular fluid is approximately equal to that of the intracellular fluid. Therefore, plasma osmolality is a guide to intracellular osmolality.

Osmolality increases with dehydration and decreases with overhydration. In normal people, increased osmolality in the blood will stimulate secretion of antidiuretic hormone (ADH). This will result in increased water reabsorption, more concentrated urine, and less concentrated blood plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.

Blood tests | Medical signs