free water clearance


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free wa·ter clear·ance

the amount of water excreted in the urine beyond that which would accompany the excreted solutes if the urine were isosmotic with plasma; it represents the loss of body water in excess of solute tending to raise body osmolality and making urine hyposmotic. Unlike other clearances, it is calculated by subtracting the osmolal clearance from the actual volume of urine excreted per minute. A negative value for free water clearance represents the amount of water that the body has reclaimed from isosmotic tubule fluid to make the urine hyperosmotic and to lower body osmolality.
References in periodicals archive ?
Higher doses of VPA-98 led to higher net fluid volume, free water clearance, serum sodium concentration, and serum osmolality, in conjunction with a significant decrease in urinary osmolality and no change in urinary sodium excretion (Hepatology 37[1]:182-91, 2003).
The volume of urine voided at each time point was recorded and aliquots were taken for measurement of osmolality (UOs) and calculation of free water clearance (C|H.sub.2~O) as defined by the function urine volume/time x (1 - UOs/POs).