Apparatus for assaying an analyte in a patient's body comprising: at least one sensor unit mountable to a region of the patient's skin comprising at least one light source that illuminates a tissue region below the skin with light that generates photoacoustic waves therein and at least one acoustic transducer that generates signals responsive to the photoacoustic waves; and a control unit configured to be mounted to a different part of the body and comprising a controller for controlling the at least one light source and at least one acoustic transducer and receiving the signals generated by the at least one acoustic transducer.