The renal clearance and infusion clearance of inulin are similar, but not identical.

Thirty-eight simultaneous renal (R-Cin) and infusion (INF-Cin) clearances of inulin were done. The equilibration period preceding the clearance studies was of at least two hours duration. The R-Cin on each subject was based on two clearance periods during which the plasma inulin 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-Cin and the INF-Cin (r = 0.976), but the INF-Cin consistently exceeded the R-Cin (mean difference = 13.8 +/- 8.8 ml/min/1.73 m2, t = 9.7163 and P = < 0.001). Complete equilibration of inulin in body fluids has been assumed when [P(in)] levels were relatively constant (variation < 10%). However, complete equilibration of inulin 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 plus 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-Cin requires complete equilibration of inulin in body fluids, a process probably requiring 12 to 15 hours or longer.