A method for measuring a glomerular filtration rate in a mammalian kidney comprises a source of reporter and marker fluorescent molecules. The fluorescent molecules are introduced into the blood stream of a mammalian subject. Over a period of time, a measurement of the intensities of the reporter and marker fluorescent molecules is taken. A ratio is calculated to determine the health of the subject's kidney. This method measures volume of plasma distribution based on a fluorescence of a marker molecule relative to a fluorescence of a reporter molecule.
