T-wave amplitude and QT interval are derived from patient cardiac signals. Then blood glucose levels are determined based on a combination of the T-wave amplitude and the QT interval. By using a combination of both T-wave-based and QT interval-based signals, blood glucose levels can be reliably detected throughout a wide range of blood glucose levels. Once the blood glucose level has been detected, the implanted device compares the blood glucose level against upper and lower acceptable bounds and appropriate warning signals are generated if the level falls outside the bounds. In one example, wherein an implantable insulin pump is additionally provided, the pump is controlled based on the detected blood glucose level to maintain glucose levels within an acceptable range. A calibration technique is also provided for determining patient-specific parameters for use in the detection of blood glucose levels.