Abstract: The present invention is directed to an apparatus (10) for reliable quantitative measurement of a fluorescent blood analyte in tissue (12) comprising: at least one light source (14), the light source (14) emitting excitation light at least at a first wavelength range between 350 nm and 450 nm to the tissue (12); a detection unit (16), the detection unit (16) measuring: a) a portion of the fluorescent light emitted by the fluorescent blood analyte excited at the first wavelength range; and b) a portion of the auto fluorescence emitted by the tissue (12); and/or c1) a portion of the remitted excitation light at the first wavelength range, and c2) a portion of the remitted light at a second wavelength range; and a control unit (18), the control unit (18) operating the light source (14) and detection unit (16). The present invention is further directed to a method for quantitative measurement of a fluorescent blood analyte in tissue (12).