Excretion of inulin and creatinine by the human kidney

Abstract

1. The excretion of inulin in man has been examined. Over the plasma concentration range used and within the experimental error of the techniques employed the rate of excretion has been found to be directly proportional to the plasma concentration.

2. The existing methods for the determination of creatinine in plasma, serum and urine have been critically examined. It is concluded that the use of Lloyd's reagent in conjunction with precipitation of serum proteins by a method which gives 'acid' filtrates, or with diluted urines, provides the most reliable method for the determination of creatinine in these fluids.

3. The excretion of creatinine in man has been examined. With the methods employed the rate of excretion has been found to depend to some extent on the plasma concentration particularly at high plasma concentrations. Within the range of plasma creatinine concentrations from 1 to 10 mg./l00 ml. the rate of excretion is independent of the plasma concentration.

4. The relationship between the clearances of inulin and creatinine has been examined. The true endogenous creatinine clearance has been found to have a variable relation to the inulin clearance, although the clearance ratio is always greater than 1.0 in normal subjects. In subjects with renal disease the clearance ratio tended to be greater than in normal subjects.

5. The possible relationship between the glomerular filtration rate and the renal clearances of inulin and creatinine have been discussed. It is concluded that there is at present no conclusive evidence to validate the acceptance of the clearance of either substance as a measure of the glomerular filtration rate. For most present day clinical purposes either may be used as an index of the glomerular filtration rate.