A system comprising: an implantable medical device, including an electrical impedance measurement circuit configured to measure at least one thoracic impedance-indicating signal characteristic, including at least a fluid status component, using information about electrical energy injected between two or more electrodes and a potential difference created thereby between the same or different two or more electrodes, and a memory circuit including a histogram comprising a plurality of histogram bins, each bin representing a subrange of thoracic impedance-indicating signal characteristic values, the memory circuit being configured for storing a value representative of the at least one thoracic impedance indicating signal characteristic into one of the histogram bins having a numerically inclusive subrange, wherein each histogram bin includes a corresponding count indicative of a number of thoracic impedance-indicating signal characteristic values detected during a particular time period and falling within the subrange of the corresponding histogram bin, and wherein the memory circuit further includes a counter circuit configured to increment, for each thoracic impedance-indicating signal characteristic value stored in a particular histogram bin, the count for the corresponding histogram bin; a comparator circuit configured to compute a deviation-using one or more histogram bins received from the memory circuit and one or more corresponding baseline histogram bins having substantially the same numerical subrange; and a processor circuit including an input to receive and use information about the deviation to compute and provide a lung fluid status indication. an implantable medical device, including an electrical impedance measurement circuit configured to measure at least one thoracic impedance-indicating signal characteristic, including at least a fluid status component, using information about electrical energy injected between two or more electrodes and a potential difference created thereby between the same or different two or more electrodes, and a memory circuit including a histogram comprising a plurality of histogram bins, each bin representing a subrange of thoracic impedance-indicating signal characteristic values, the memory circuit being configured for storing a value representative of the at least one thoracic impedance indicating signal characteristic into one of the histogram bins having a numerically inclusive subrange, wherein each histogram bin includes a corresponding count indicative of a number of thoracic impedance-indicating signal characteristic values detected during a particular time period and falling within the subrange of the corresponding histogram bin, and wherein the memory circuit further includes a counter circuit configured to increment, for each thoracic impedance-indicating signal characteristic value stored in a particular histogram bin, the count for the corresponding histogram bin; a comparator circuit configured to compute a deviation-using one or more histogram bins received from the memory circuit and one or more corresponding baseline histogram bins having substantially the same numerical subrange; and a processor circuit including an input to receive and use information about the deviation to compute and provide a lung fluid status indication.