Comparison of the Constant Infusion and Urine Collection Techniques for the Measurement of Renal Function 1

For any exogenous substance which is neither metabolized, stored nor excreted otherwise than in the urine, the rate of excretion must be equal to the rate of infusion under conditions where its plasma level and volume of distribution are constant.This steady state, as indicated by a constant plasma level, will be designated infusion equilibrium.A simplified method for the measurement of glomerular filtration rate and the effective renal plasma flow based upon the above principle has been described elsewhere (1).In the application of the principle, the rate of infusion, IV (where I is the concentration in mg. per ml. of infusion fluid and V is the volume of fluid injected in ml. per minute), can be substituted for the rate of excre- tion, UV, in the clearance calculation, i.e., IV/P = UV/P.The procedure, therefore, obviates the necessity of bladder catheterization and the errors inherent in urine collection.The. study presented here embodies further ob- servations, obtained under normal and abnormal circumstances, on the correlation of inulin and p-aminohippuric acid clearances as measured by urine collections with those measured simultane- ously by the infusion technique, and on the appli- cation of the infusion technique to the measure- ment of p-aminohippuric acidTm.EXPERIMENTAL PROCEDURE Glomerular filtration rate and renal plasma flow were estimated from the clearances (2, 3) of inulin and p- aminohippurate (PAH) 2 by the usual urine collection technique involving the use of a multi-eyed catheter and 1 This investigation was supported by a grant from the Life Insurance Medical Research Fund.2The authors are indebted to the Medical Research Division of Sharp and Dohme, Inc., of Glenolden, Pa., for generous supplies of sodium-p-aminohippurate in ampules.