Evaluation of Renal Concentrating Ability

In addition to gross assessment of renal dilution or concentrating ability (as described above), quantitative evaluation of either osmolar clearance, freewater clearance (Ch2o), or solute-free water clearance (ECH2o) may add additional sensitivity to the evaluation of these functions. 

Freewater clearance (Ch2o) provides an estimate of the amount of urine being excreted in excess that needed to clear solutes, and is calculated traditionally as follows  

More accurate assessment of the ability of the kidney to appropriately regulate plasma tonicity and/or respond appropriately to antidiuretic hormone (ADH) can be made by calculation of electrolyte-free water clearance (ECh2o)- This formula takes into account only water needed to excrete excess effective os-molytes (in general, monovalent electrolytes and their associated anions) and is calculated as follows  

Ragan HA, Weller RE (1999) Markers of renal function and injury. In Loeb WF, Quimby FW (eds) The Clinical Chem- 

Nguyen MK, Kurtz I (2005) Derivation of a new formula for calculating urinary electrolyte-free water clearance based on the Edelman equation. Am J Physiol Renal Physiol 288 F1-F7 

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