1. A method of detecting tissue oedema in a subject, the method including, in a processing system: a) determining a measured impedance for first and second body segments; b) for each body segment, and using the measured impedance, determining an index indicative of a ratio of the extra-cellular to intra-cellular fluid; c) determining an index ratio based on the index for the first and second body segments; d) determining the presence, absence or degree of tissue oedema based on the index ratio.