An implantable medical device system including an optical sensor monitors for the presence of overgrowth on the sensor by sensing light scattered by a measurement volume, the sensed light corresponding to a first wavelength, and deriving an overgrowth metric in response to the sensed light. The overgrowth metric is correlated to the presence of overgrowth on the sensor and is compared to a predetermined threshold. The presence of overgrowth on or near the sensor is detected in response to the overgrowth metric crossing the threshold.