It could be confirmed that the endogenic creatinine clearance overestimates the GFR. This overestimation increases with the restriction of the glomerular filtration. 2. By the biphasic pump-regulated permanent infusion of inulin and PAH a constant equilibrium was reached and the clearance was calculated according to the classical formula (output method) as well as from the addition of the test substance (input method) and compared. 3. The comparison of the two clearance techniques results in significantly higher value for inulin as well as for PAH in the input technique. 4. By clearance equations the relation of input and output clearance can be made. The correlation coefficient for the different investigation techniques for the estimation of the inulin clearance was 0.83, for the PAH-clearance 0.88.
