A medical system for determining prospective heart failure event risk, the system comprising: processing means for determining whether a patient is at risk of hypervolemia, the processor configured to: (a) acquire from a device memory a patient's absolute intrathoracic impedance data; (b) determine an average of the intrathoracic impedance data over a pre-specified time period; and (c) determine whether the average of the intrathoracic impedance data is out-of-range from one of a first and second range, the first range being a higher value boundary of intrathoracic electrical impedance and the second range being a lower value boundary of intrathoracic electrical impedance; and characterized by the processor further being configured to: d) in response to step (c), determine level of heart failure event risk based upon whether the moving average of impedance is outside of the targeted impedance range, determining the heart failure event risk comprising calculating a patient risk score based on the degree to which the moving average of the intrathoracic impedance is out-of-range and classifying the risk as high, medium or low based on the score; wherein the patient is determined to exhibit excessive fluid in response to determining that the average of intrathoracic impedance data is outside of the target impedance range and extends beyond the first range, and wherein the patient is determined to be dehydrated in response to determining the average of the intrathoracic impedance data is outside of the target impedance range and extends away from the second range; and wherein the processing means is further configured to: in response to step (c), perform one of automatically adjusting therapy delivered to a patient and to generate a notification to one of a patient and health care worker, based on the risk classification. (a) acquire from a device memory a patient's absolute intrathoracic impedance data; (b) determine an average of the intrathoracic impedance data over a pre-specified time period; and (c) determine whether the average of the intrathoracic impedance data is out-of-range from one of a first and second range, the first range being a higher value boundary of intrathoracic electrical impedance and the second range being a lower value boundary of intrathoracic electrical impedance; and characterized by the processor further being configured to: d) in response to step (c), determine level of heart failure event risk based upon whether the moving average of impedance is outside of the targeted impedance range, determining the heart failure event risk comprising calculating a patient risk score based on the degree to which the moving average of the intrathoracic impedance is out-of-range and classifying the risk as high, medium or low based on the score; wherein the patient is determined to exhibit excessive fluid in response to determining that the average of intrathoracic impedance data is outside of the target impedance range and extends beyond the first range, and wherein the patient is determined to be dehydrated in response to determining the average of the intrathoracic impedance data is outside of the target impedance range and extends away from the second range; and wherein the processing means is further configured to: in response to step (c), perform one of automatically adjusting therapy delivered to a patient and to generate a notification to one of a patient and health care worker, based on the risk classification. the patient is determined to exhibit excessive fluid in response to determining that the average of intrathoracic impedance data is outside of the target impedance range and extends beyond the first range, and wherein the patient is determined to be dehydrated in response to determining the average of the intrathoracic impedance data is outside of the target impedance range and extends away from the second range; and wherein the processing means is further configured to: in response to step (c), perform one of automatically adjusting therapy delivered to a patient and to generate a notification to one of a patient and health care worker, based on the risk classification.