Evaluation of urine osmolality and water clearance are important in assessment of water excretion or conservation by the kidney. There are 3 clearances that are used:
• osmolal clearance: the hypothetical volume of plasma cleared of solute per minute, expressed as mL/min
• free water clearance: the hypothetical rate at which water is excreted during production of hypotonic urine in excess of that required for excretion of the solute load in isosmolal urine
• negative free water clearance: the rate at which water is reabsorbed from tubular fluid during the production of hypertonic urine; it is the volume of water conserved as compared with the volume needed to excrete the solute load in isosmolal urine.
