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”), an external computing device, and a backend computing system. The IMD determines a physiologic parameter value within a patient's body, and communicates the physiologic parameter value outside the patient's body, for example, to the external computing device. Further, the external computing device receives the physiologic parameter from the IMD and communicates it to the backend computing system. The backend computing system receives the physiologic parameter value and obtains an ambient condition value outside the body. The backend computing system then calculates a relative physiologic parameter value from the ambient condition value and the physiologic parameter value, and in some embodiments, stores the relative physiologic parameter value in a storage location, such as a memory or database.