Free water clearance

It is the amount of water excreted in urine beyond the quantity that is necessary to make urine isosmotic with plasma. For example,  in case of hyposmotic urine, more water is excreted than is necessary to make urine isosmotic with plasma and this constitues positive free water clearance.

Free water clearance = Urine excreted in one minute – osmolal clearance

where;

Osmolal clearance = urine excreted in one minute X osmolality of urine/osmolality of plasma.