Discrepancies between transcutaneous and estimated glomerular filtration rates in rats with chronic kidney disease

Accurate assessment of the glomerular filtration rate (GFR) is crucial for researching kidney disease in rats. Although validation of methods that assess GFR is crucial, large-scale comparisons between different methods are lacking. Both transcutaneous GFR (tGFR) and a newly developed estimated GFR (eGFR) equation by our group provide a low-invasive approach enabling repeated measurements. The tGFR is a single bolus method using FITC-labeled sinistrin to measure GFR based on half-life of the transcutaneous signal, whilst the eGFR is based on urinary sinistrin clearance. Here, we retrospectively compared tGFR, using both 1- and 3- compartment models (tGFR_1c and tGFR_3c, respectively) to the eGFR in a historic cohort of 43 healthy male rats and 84 male rats with various models of chronic kidney disease. The eGFR was on average considerably lower than tGFR-1c and tGFR-3c (mean differences 855 and 216 μL/min, respectively) and only 20 and 47% of measurements were within 30% of each other, respectively. The relative difference between eGFR and tGFR was highest in rats with the lowest GFR. Possible explanations for the divergence are problems inherent to tGFR, such as technical issues with signal measurement, description of the signal kinetics, and translation of half-life to tGFR, which depends on distribution volume. The unknown impact of isoflurane anesthesia used in determining mGFR remains a limiting factor. Thus, our study shows that there is a severe disagreement between GFR measured by tGFR and eGFR, stressing the need for more rigorous validation of the tGFR and possible adjustments to the underlying technique.

Improved kinetic model for the transcutaneous measurement of glomerular filtration rate in experimental animals

Transcutaneous measurement of the glomerular filtration rate (_tGFR) is now frequently used in animal studies. _tGFR 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 (_tGFR_3cp.b.m), _tGFR 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 _tGFR measurements in the same animals, showed higher precision when _tGFR_3cp.b.m was compared with the 1-compartment _tGFR_1cp model.