Osmolal and Water Clearance


Nephrology, Urology, Obstetrics & Gynecology, Pedatrics, Genetics
Clinical Laboratory




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.

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