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