renal clearance ratio 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: c l e a r a n c e r a t i o o f x = c x c i n x is the analyte substance cx is the renal plasma clearance of x cin is the renal plasma clearance of inulin. diagram showing the basic physiologic mechanisms of the kidney 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.