Comparison and reproducibility of plasma clearance of exogenous creatinine, exo-iohexol, endo-iohexol, and 51Cr-EDTA in young adult and aged healthy cats

Background: Important characteristics determining the usefulness of a method for glomerular filtration rate (GFR) measurement are convenience, availability, and reproducibility. Hypothesis: The use of different plasma clearance methods could lead to different results and differences in reproducibility. Animals: Twelve healthy cats: 6 young adult cats (age 7-12 months), and 6 aged cats (age 9-12 years) were included in this study. Methods: A cross-over design was used to compare the plasma clearance of exogenous creatinine (PECCT), exo-iohexol (PexICT), endo-iohexol (PenICT), and chromium-51 ethylenediaminetetraacetic acid (Cr-51-EDTA), and to investigate reproducibility of these methods. Cats of different ages were included to determine if differences in GFR in young adult versus aged cats would be detected with these methods. The PECCT, PexICT, and PenICT were performed in a combined manner. Plasma data were subjected to noncompartmental (creatinine, exo-iohexol, and endo-iohexol) or bicompartmental ((51)CrEDTA) analysis with a statistical moment approach. Area under the concentration-time curve was calculated using the trapezoidal rule with extrapolation to infinity. Statistical analyses were carried out using a random effects model. Results: Globally, the 4 methods differed significantly (P <.0001) in GFR assessment. Clearance of exo-iohexol and chromium-51 ethylenediaminetetra acetic acid (Cr-51 -EDTA) showed the highest and lowest reproducibility, respectively. Only plasma clearance of creatinine differed significantly between young adult and aged cats. Conclusion and Clinical importance: We found considerable differences in reproducibility of different GFR measurements. These findings should be taken into account not only in practice but also in future studies involving GFR measurement