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.^[1][2] It is defined by following equation:

${\displaystyle clearance\ ratio\ of\ X={\frac {C_{x}}{C_{in}}}}$

• X is the analyte substance
• C_x is the renal plasma clearance of X
• C_in 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.^[3] It is easier to use creatinine rather than inulin as the comparator as creatinine is produced by the body, whereas inulin has to be intravenously infused.