A system for detecting substances

The present invention relates to a method and system for the in vivo determination of the presence and/or concentration of biological and/or chemical substances in body lumens. The system of the invention comprises a solid support, the support being inserted into a body lumen and having immobilized thereon at least one reactant capable of reacting with the substance resulting in an optical change; and a detecting unit, in communication with the support, capable of detecting a reaction resulting in an optical change between the reactant and the substance.