An apparatus for measuring the concentration of an analyte in a sample extracted from the skin (10) of a patient comprises a collection chamber (3) adjacent to the skin; a radiation source (7) configured to irradiate the sample in the collection chamber (3); a reflector (4) configured to reflect incident radiation back through the sample; and a sensor (11) configured to measure the radiation emitted from the sample. The reflector (4) may shield the skin (10) from the incident radiation and have a convoluted or perforated shape to permit sufficient diffusion of the extracted analyte to all parts of the surface of the reflector from its edges. The reflector (4) may be a reflective surface of reverse iontophoresis electrodes (9) used to extract the sample from the skin. In a further apparatus the reflector (4) is implanted beneath an outer layer of the skin (10) so that instead of being extracted, the sample can be measured in vivo. An implanted reflector (4) may be formed from an assemblage of reflective particles. A mask (5) may be provided to prevent the incident radiation bypassing the reflector (4).