A portable device for detecting a change in concentration of a blood analyte e.g. glucose in a subject comprises a light source 302 such as a laser adapted to irradiate a portion of the skin of the subject with incident light such that said light undergoes Raman scattering by a blood analyte present in a dermal tissue layer and a detection system 308 adapted to detect Raman scattered light emitted from said dermal tissue layer. The detection system 308 comprises a first (e.g. notch) filter 309 adapted to reject reflected incident light and a second (e.g. bandpass) filter 310 adapted to filter the Raman scattered light such that a selected portion of the Raman scattered light is transmitted to the detector 311. A processor is operably coupled to the detector 311, and is adapted to determine the change in intensity of the Raman scattered light and determine the change in concentration of the analyte therefrom. The device may be adapted to be worn about the wrist of the subject.
