Renal excretion of creatinine in the rat

The exogenous creatinine clearance was measured in rats using a technique of constant intravenous infusion, and was compared with simultaneous measurement of the radioinulin clearance. Creatinine/inulin clearance ratios were significantly higher than unity at plasma concentrations of creatinine less than 150 mg%, indicating tubular secretion of creatinine. At higher plasma creatinine concentrations (150–500 mg%) the clearance ratios were depressed and did not differ significantly from one. It is concluded that at higher plasma creatinine concentrations the exogenous creatinine clearance may be used as an estimate of glomerular filtration rate.