Renal clearance ratio
The renal clearance ratio or fractional excretion is a relative measure of the speed at which a constituent of urine passes through the kidneys. It is defined by following equation:

 X is the analyte substance
 Cx is the renal plasma clearance of X
 Cin is the renal plasma clearance of inulin.

Creatinine is sometimes used instead of inulin as the reference substance; for example, the calcium-creatinine clearance ratio is used in an attempt to distinguish between different causes of a high plasma calcium concentration. It is easier to use creatinine rather than inulin as the comparitor as creatinine is produced by the body, whereas inulin has to be intravenously infused.

See also
 Clearance (pharmacology)
 Kt/V

References

Nephrology