Abstract: A method is provided for measuring an analyte in a sample comprising adding substantially transparent particles to a sample in solution or suspension, said particles having an affinity for said analyte; fractionating the particles from the solution or suspension to form a particle-rich fraction and a substantially particle-free fraction; optically reading the particle-rich fraction at a first and a second wavelength; optically reading the substantially particle-free fraction at at least the first wavelength; and correlating the readings through the particle-rich fraction and the substantially particle-free fraction of the sample, with similar measurements in a particle-containing "blank" to obtain a quantitative determination of the analyte originally present in the sample.