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Transcutaneous measurement of the glomerular filtration rate ( t GFR) is now frequently used in animal studies. t GFR allows consecutive measurements on the same animal, including multiple measurements on a daily basis, because no blood sampling is required. Here we derive and validate a novel kinetic model for the description of transcutaneously measured FITC-Sinistrin excretion kinetics. In contrast to standard 1- to 3-compartment models, our model covers the complete kinetic, including injection and distribution of the tracer in the plasma compartment. Because the model describes the complete progression of the measurement, it allows further refinement by correcting for baseline shifts observed occasionally during measurement. Possible reasons for shifts in the background signal include photo bleaching of the skin, autofluorescence, changes of physiological state of the animals during the measurements, or effects arising from the attachment of the measurement device. Using the new 3-compartment kinetic model with modulated baseline ( t GFR 3cp.b.m ), t GFR measurements in rats can reach comparable precision as those from GFR measurements assessed using a gold standard technique based on constant infusion of a tracer. Moreover, the variability of simultaneous (parallel) measurements, as well as repeated t GFR measurements in the same animals, showed higher precision when t GFR 3cp.b.m was compared with the 1-compartment t GFR 1cp model., (Copyright © 2016 International Society of Nephrology. Published by Elsevier Inc. All rights reserved.)

Constant infusion clearance techniques using exogenous renal markers are considered the gold standard for assessing the glomerular filtration rate. Here we describe a constant infusion clearance method in rats allowing the real-time monitoring of steady-state conditions using an automated closed-loop approach based on the transcutaneous measurement of the renal marker FITC-sinistrin. In order to optimize parameters to reach steady-state conditions as fast as possible, a Matlab-based simulation tool was established. Based on this, a real-time feedback-regulated approach for constant infusion clearance monitoring was developed. This was validated by determining hourly FITC-sinistrin plasma concentrations and the glomerular filtration rate in healthy and unilaterally nephrectomized rats. The transcutaneously assessed FITC-sinistrin fluorescence signal was found to reflect the plasma concentration. Our method allows the precise determination of the onset of steady-state marker concentration. Moreover, the steady state can be monitored and controlled in real time for several hours. This procedure is simple to perform since no urine samples and only one blood sample are required. Thus, we developed a real-time feedback-based system for optimal regulation and monitoring of a constant infusion clearance technique.

Determination of the urinary or plasma clearance of exogenous renal markers, such as inulin or iohexol, is considered to be the gold standard for glomerular filtration rate (GFR) measurement. Here, we describe a technique allowing determination of renal function based on transcutaneously measured elimination kinetics of fluorescein isothiocyanate (FITC)-sinistrin, the FITC-labeled active pharmaceutical ingredient of a commercially available marker of GFR. A low cost device transcutaneously excites FITC-sinistrin at 480  nm and detects the emitted light through the skin at 520  nm. A radio-frequency transmission allows remote monitoring and real-time analysis of FITC-sinistrin excretion as a marker of renal function. Due to miniaturization, the whole device fits on the back of freely moving rats, and requires neither blood sampling nor laboratory assays. As proof of principle, comparative measurements of transcutaneous and plasma elimination kinetics of FITC-sinistrin were compared in freely moving healthy rats, rats showing reduced kidney function due to unilateral nephrectomy and PKD/Mhm rats with cystic kidney disease. Results show highly comparable elimination half-lives and GFR values in all animal groups. Bland-Altman analysis of enzymatically compared with transcutaneously measured GFR found a mean difference (bias) of 0.01 and a -0.30 to 0.33 ml/min per 100 g body weight with 95% limit of agreement. Thus, with this device, renal function can be reliably measured in freely moving rats eliminating the need for and influence of anesthesia on renal function.