Apparatus and methods for monitoring and indexing glucose consumption are disclosed. Embodiments of the present invention provide for accurate measurement of small differences in blood glucose values at various points in the body. These accurate measurements can be used to monitor glucose uptake in specific organs of the body such as the brain or heart. The apparatus includes an upstream intravascular glucose sensor and a downstream intravascular glucose sensor. In some embodiments, substantially simultaneous fluid communication of a calibration fluid to both glucose sensors reduces bias between the sensors. A processor is used to calculate and optionally display blood glucose consumption. Oxygen consumption can also be determined and used to determine and display an index value that is indicative of any mismatch between the two. In some embodiments, insulin and/or glucose infusion can be controlled by the system.