THE RENAL CLEARANCE AND INFUSION CLEARANCE OF INULIN ARE SIMILAR, BUTNOT IDENTICAL

Thirty-eight simultaneous renal (R-C(in)) and infusion (INF-C(in)) cle arances of inulin were done. The equilibration period preceding the cl earance studies was of at least two hours duration. The R-C(in) on eac h subject was based on two clearance periods during which the plasma i nulin concentration ([P(in)]) varied by 1.0 mg/dl or less and the rate of inulin excretion by less than 10%. There was excellent correlation between the R-C(in) and the INF-C(in) (r = 0.976), but the INF-C(in) consistently exceeded the R-C(in) (mean difference = 13.8 +/- 8.8 ml/m in/1.73 m2, t = 9.7163 and P = < 0.001). Complete equilibration of inu lin in body fluids has been assumed when [P(in)] levels were relativel y constant (variation < 10%). However, complete equilibration of inuli n would not be present, even with relatively constant P(in) levels, if the rate of infusion of inulin were equal to the rate of excretion pl us the rate of penetration of inulin into less permeable components of the extracellular fluid compartment (that is, dense connective tissue solids). Estimation of glomerular filtration rate using the INF-C(in) requires complete equilibration of inulin in body fluids, a process p robably requiring 12 to 15 hours or longer.