Disclosed are systems and methods of determining the amount of a substance in a living organism. In one embodiment, the method comprises: applying an electrical signature signal to the living organism, wherein the electrical signature signal corresponds to a predetermined amount of the substance; measuring the response of the living organism to the applied signature signal; and determining whether an elevated response has resulted from applying the electrical signature signal, if so, then determining the amount of the substance in the living organism from the predetermined amount of the substance.