A method of evaluating or monitoring the medical state of a subject comprising the steps of: providing a radio frequency interrogation interference signal from a subject, the radio frequency interrogation interference signal being low frequency components of reflections of a radio interrogation signal transmitted into the thorax of the subject; and determining at least one cardiac or respiratory characteristic of the subject from radio frequency interrogation interference signal. The characteristic can be compared to predetermined values to assess current state or monitored for changes over time to assess change in condition of the subject.