One embodiment of the present invention relates to a system for deriving physiologic measurement values that are relative to ambient conditions. In one embodiment, the system comprises an implantable medical device (“IMD”) and an external computing device. The IMD is operable to determine an absolute physiologic parameter value within a patient's body, and communicate the absolute physiologic parameter value outside the patient's body, for example, to the external computing device. Further, the external computing device is operable to receive the absolute physiologic parameter from the IMD and obtain an ambient condition value outside the body that can affect the absolute physiologic parameter value. The external computing device then calculates a relative physiologic parameter value from the ambient condition value and the absolute physiologic parameter value.