A method (200) for determining a concentration of an analyte in a fluid or fluid sample, comprises: providing (201) a SERS substrate comprising receptor molecules (107) capable of binding competitor molecules (106); contacting (202) the SERS substrate (102) with a fluid (sample) comprising analyte (108) and such competitor molecules (106); radiating (203) the SERS substrate (102) with a light source while measuring a SERS signal; and determining (205) a concentration of the analyte (108) based on the measured signal level.A corresponding device and system are also provided.