An apparatus for quantitative measurement of a fluorescent blood analyte in tissue comprising: at least one light source for emitting excitation light in a first wavelength range between 350 nm and 450 and light in a second wavelength range where the blood absorption is substantially smaller to the tissue; a detection unit for measuring: a) a portion of fluorescent light emitted by the fluorescent blood analyte excited by illumination of the tissue with light in the first wavelength range; and at least one of b) a portion of autofluorescence emitted by the tissue; or c) a portion of remitted excitation light in the first wavelength range and a portion of remitted light in said second wavelength range; and a control unit for operating the light source and detection unit; wherein the apparatus further comprises a processor configured to determine a parameter from at least one detected intensity of said portion of the autofluorescence emitted by the tissue in a wavelength range between 530 and 600 nm, especially at 580 nm, and to identify measurements and/or tissue sites where the detected amount of fluorescent light is a quantitative measure of the fluorescent blood analyte based on a comparison of the determined parameter with a predetermined threshold; and/or a processor configured to determine a parameter from the detected intensity of the remitted excitation light in the first wavelength range and the detected intensity of the remitted light in the second wavelength range, such as the ratio of the remitted light at the first wavelength range and the second wavelength range, and to identify measurements and/or tissue sites, where the detected amount of fluorescent light is a quantitative measure of the fluorescent blood analyte based on a comparison of the determined parameter with a predetermined threshold. at least one light source for emitting excitation light in a first wavelength range between 350 nm and 450 and light in a second wavelength range where the blood absorption is substantially smaller to the tissue; a detection unit for measuring: a) a portion of fluorescent light emitted by the fluorescent blood analyte excited by illumination of the tissue with light in the first wavelength range; and at least one of b) a portion of autofluorescence emitted by the tissue; or c) a portion of remitted excitation light in the first wavelength range and a portion of remitted light in said second wavelength range; and a) a portion of fluorescent light emitted by the fluorescent blood analyte excited by illumination of the tissue with light in the first wavelength range; and at least one of b) a portion of autofluorescence emitted by the tissue; or c) a portion of remitted excitation light in the first wavelength range and a portion of remitted light in said second wavelength range; and a control unit for operating the light source and detection unit; wherein the apparatus further comprises a processor configured to determine a parameter from at least one detected intensity of said portion of the autofluorescence emitted by the tissue in a wavelength range between 530 and 600 nm, especially at 580 nm, and to identify measurements and/or tissue sites where the detected amount of fluorescent light is a quantitative measure of the fluorescent blood analyte based on a comparison of the determined parameter with a predetermined threshold; and/or a processor configured to determine a parameter from the detected intensity of the remitted excitation light in the first wavelength range and the detected intensity of the remitted light in the second wavelength range, such as the ratio of the remitted light at the first wavelength range and the second wavelength range, and to identify measurements and/or tissue sites, where the detected amount of fluorescent light is a quantitative measure of the fluorescent blood analyte based on a comparison of the determined parameter with a predetermined threshold.