Systems and methods for disambiguation of posture

A method of operating a medical device system comprising:

• measuring position data using an accelerometer;
• making a preliminary posture determination of a patient using the position data measured by the accelerometer;
• when the preliminary posture determination does not indicate a recumbent posture, then:,
• measuring chemical data using an optical chemical sensor;
• and
• normalizing the chemical data by correcting the chemical data in a manner that is proportional to the position data indicated by the accelerometer;
• and
• making a final posture determination of the patient using the position data measured by the accelerometer and the normalized chemical data measured by the optical chemical sensor;
• wherein the accelerometer and the optical chemical sensor are disposed within a housing of an implantable medical device.