StatPearls-/-ä 2024[Jan]; ä (ä): ä PMID33620841show ga
Osmolality indicates the concentration of all the particles dissolved in body fluid. It is routinely measured in clinical laboratories for the differential diagnosis of disorders related to hydrolytic balance regulation, renal function, and small-molecule poisonings. Serum and urine osmolality tests are usually measured together to be compared and reach the diagnosis of any disease that influences osmolality. Serum osmolality is affected by the concentration of blood chemicals like chloride, sodium (Na), proteins, bicarbonate, and glucose. The blood urea nitrogen (BUN) measurement is important for calculating serum osmolality. Specific therapies and toxins that affect an individual's fluid balance should also be evaluated with serum osmolality. The 1975 Dorwart and Chalmers formula, serum osmolality = 1.86(Na) + (glucose/18) + (BUN/2.8) + 9, had been often used to calculate plasma osmolality. In 1976, Smithline and Gardner proposed to use serum osmolality = 2(Na) + glucose/18 + BUN/2.8 as a simpler formula. In 1987, Worthley et al. concluded that the best formula was the simple Smithline-Gardner formula, where the plasma concentrations are measured in mmol/l. Many other formulas have been developed and used over the years, but the simple Smithline-Gardner formula remains the most useful. Some authors argue that the formula 1.86(Na+K)+1.15(Glu/18)+(Urea/6)+14 is the most precise of them. The normal serum osmolality should range from 275 to 295 mOsm/kg. Water normally flows from the compartment of low osmolality to the compartment of high osmolality; this only occurs if the membrane between the two compartments is permeable to water. When water moves between plasma and intracellular compartments, the movement direction depends on both compartments' osmolalities. For example, if a cell is in a relatively hyperosmolar solution, fluid will move out of the cell towards the highly concentrated compartment to reach homeostasis. As a result, the cell will shrink.
StatPearls-/-ä 2024 ; ä (ä): ä
