THE USE OF INULIN CLEARANCE AS A MEASURE OF GLOMERULAR FILTRATION

Abstract

1. Following a single intravenous injection of inulin, values for the renal clearance of inulin (Cin = µν/P) show a steady decline, with diminishing values of plasma inulin concentrations. 2. Employing values of P 6 minutes before the mid‐point of the clearance period, in place of values 2½‐minutes before the mid‐point, does not eliminate this phenomenon. 3. Employing the formula Cin = µν/P, the fall in values for clearance is due to the fact that the renal excretion of inulin (µν) is not proportional to the plasma level (P), since the imposition of a linear relationship between the two variables results in lines which deviate significantly from the point of origin. 4. The lack of proportionality is shown not to be due to the sampling of venous blood as an estimate of arterial plasma inulin concentrations. 5. The dependence of the magnitude of the renal clearance of inulin upon the plasma concentration of inulin is found also in three subjects using the continuous infusion method of administration. This excludes the possibility that the results obtained, following a single injection of inulin, are distortions of a simple proportional relationship consequent upon the dynamics of distribution of inulin in the body following a single injection. 6. The results of the experiments, and those of other workers, are held to suggest strongly that, in man, inulin is reabsorbed by the tubules to a significant extent. Hence the use of inulin clearance as a measure of glomerular filtration rate involves a systematic error, which is estimated to be about 15 per cent. under the usual circumstances. © 1950 The Physiological Society