The present invention relates to a device (10), system (1) and method (200) for use in blood oxygen saturation measurement of a subject. To enable blood oxygen saturation measurements with improved reliability, a processing device (10) is presented comprising an input (11) for receiving first and second detection data of a tissue region of the subject, said first detection data being data acquired over time by detecting radiation at a first wavelength (λ1) and at a second wavelength (λ2) received from said tissue region; said second detection data being data acquired over time by detecting radiation at the first wavelength and at the second wavelength received from said tissue region in response to coherent light at the first wavelength and coherent light at the second wavelength being emitted towards the tissue region; a PPG unit (12) for deriving, from said first detection data, a first PPG signal indicative of an absorption of light within the tissue region at the first wavelength, and a second PPG signal indicative of an absorption of light within the tissue region at the second wavelength; a flow unit (13) for deriving, from said second detection data, a first flow signal indicative of a flow of light scattering particles within the tissue region probed at the first wavelength, and a second flow signal indicative of a flow of light scattering particles within the tissue region probed at the second wavelength; and a processing unit (14) for correcting said PPG signals based on said flow signals and/or for providing a feedback signal based on a comparison of the first and second flow signals.