A system (1) for determining blood component concentration in vivo, the system (1) comprising: a probe (3) having an optical component and an electrical component, said optical component comprising a light source (9) for illuminating tissue (5) of a subject, said tissue (5) including a light absorbing blood component of interest, and a light detector (11) configured to detect light that has been emitted by said source (9) and has passed through said tissue (5) said electrical component comprising electrodes (13, 15) for applying an electric field across said tissue (5) and a control module (4) configured to receive signals from said light detector (11) that are representative of the intensity of light detected by the detector (11) and to receive signals from said electrodes (13, 15) that are representative of the capacitance of said tissue (5) wherein the amplitude of said signals varies periodically with the subjects cardiac cycle, and said control module (4) comprises a processor (35) operable to determine from said signals the concentration of said blood component in said tissue (5).
