The relationship between the clearance and the plasma concentration of inulin in normal man.

Shannon and Smith (1) found that in normal man, the rate of excretion of inulin was directly proportional to its concentration in plasma when the latter varied from 50 to 400 mg. per cent. Later Miller, Alving, and Rubin (2). extended these observations to plasma concentrations of inulin between 3 and 96 mg. per cent and reached similar conclusions. Both groups considered that this constituted important evidence that inulin clearance measured the rate of glomerular filtration. Recently, Ferguson, Olbrich, Robson, and Stewart (3) presented data showing that the rate of excretion of inulin was not directly proportional to its concentration in plasma, and that the clearance of inulin was dependent upon the concentration of this substance in plasma. In view of the significance generally attached to the clearance of inulin in renal physiological studies and in view of the critical importance of precise estimation of glomerular filtration rate, it was decided to reinvestigate this problem.