The smallest animals used in research with our tGFR Monitors have been 3 weeks of age with a body weight of ~10g.1 The only limitation here is the size of the device, which needs to fit on the animals back without placing it on the spine, as this can lead to strong motion artifacts.
Regarding larger animals, there is no limitation in size. However, you need to keep in mind, that in larger animals the excretion of FITC-Sinistrin is slower, thus, longer measurement times are necessary, e.g. 3hrs for dogs and 4hrs for pigs. Also, as FITC-Sinistrin is dosed per body weight the absolute amount of FITC-Sinistrin which is needed for those larger animals can become a limiting factor.