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

Abstract

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.

Determination of the Glomerular Filtration Rate by Identification of Sinistrin Kinetics

“…As has been demonstrated and as was reported recently (36,37), steady-state techniques generally need much more time for the establishment of stationary concentration values than is suggested by traditional recipes. This is due to the difficulty in estimating the glomerular filtration rate from endogenous creatinine levels via a general statistical regression relation in the individual case (38).…”

Measurement and Estimation of GFR in Children and Adolescents

“…Van Rossum et al (20) compared the plasma clearance of inulin using a single injection method with the continuous infusion method in 24 pediatric patients with a median GFR of 42 ml/min per 1.73 m 2 . The advantage of the single injection method is that it takes about 4 h compared with a much longer period of time to reach a complete equilibrium of inulin when using a continuous infusion (27). Using the single injection method (20), inulin was infused, and blood samples were taken at 0, 10, 30, 90, and 240 min after injection.…”

Screening of Renal Function in Patients with Chronic Kidney Disease

“…The GFR (in millilitres per minute) may be calculated by dividing the rate of the infusion (in milligrams per minute) by the plasma concentration (in milligrams per millilitre). While this method gives reproducible results [16,17], it is hard to achieve a constant plasma inulin concentration, and small changes in inulin concentration can significantly affect the calculated GFR [10]. Furthermore, animal and human studies demonstrate that complete equilibration of inulin in body fluids, necessary for accurate GFR calculation, can take several hours to achieve [17,18].…”

“…Furthermore, animal and human studies demonstrate that complete equilibration of inulin in body fluids, necessary for accurate GFR calculation, can take several hours to achieve [17,18]. As a result, serum inulin concentrations are often lower than expected, which in turn leads to an overestimation of GFR, frequently exceeding urinary inulin clearance by ∼5-10 ml min −1 [17,19]. While inulin continues to be useful particularly for mechanistic studies, especially when coupled with measurement of renal clearance of para-aminohippuric acid (PAH) to assess renal plasma flow [20,21], it is seldom used nowadays, partly due to the complex methodology described above but also as a result of limitations in the supply of this agent.…”

“…While this method gives reproducible results [16,17], it is hard to achieve a constant plasma inulin concentration, and small changes in inulin concentration can significantly affect the calculated GFR [10]. Furthermore, animal and human studies demonstrate that complete equilibration of inulin in body fluids, necessary for accurate GFR calculation, can take several hours to achieve [17,18]. As a result, serum inulin concentrations are often lower than expected, which in turn leads to an overestimation of GFR, frequently exceeding urinary inulin clearance by ∼5-10 ml min −1 [17,19].…”