A method and apparatus for determining physiological data related to an animal, such as kidney diagnostics data, is provided. The method includes injecting a mixture of a first and a second molecule into an animal (e.g., a human patient), determining a molecular ratio of the molecules, and determining the physiological data based on the molecular ratio. The apparatus includes a number of finger receiving apertures, a light generation circuit, a light detection circuit, a pulse counting circuit, and a user interface.