Analyte content in a cell free portion of a body fluid, such as blood, is optically determined without centrifugation or other preliminary steps for separating the cell free portion from the body fluid. A channel is configured for containing a flowing sample of the body fluid along an optical boundary. The channel is configured so that a cell free layer of the fluid naturally forms along the boundary of the channel which coincides with the optical boundary. A light source is directed onto the optical boundary at an angle selected to generate total reflection from the boundary and to generate an evanescent field across the boundary in the cell free layer of fluid. A light detector is configured to detect absorption of the light in the evanescent field. The light source and light detector are matched to the wavelength range of an absorption peak of the analyte being detected.
