A method and an apparatus for detecting a change of the fluid status or determining the fluid status of an individual are disclosed. The method comprises the following steps: determining a change of the body parameter (ΔRBV) of the individual during a first treatment session (201); determining a first fluid status of the individual (202); calibrating the determined change of the body parameter (ΔRBV) based on the first fluid status (205); determining the change of the body parameter (ΔRBV) of the individual during at least one further treatment session (207); and deriving a fluid status or a change of fluid status individual from the change of the body parameter (ΔRBV) (208).