Method of quantifying an analyte in a liquid medium by means of magnetic particles functionalized by specific ligands of the analyte being quantified, consisting in applying a magnetic field to a liquid medium for a time period several times shorter than that of the reaction between the analyte being quantified and the ligands of the magnetic particles, in measuring the optical density of the liquid medium after applying the magnetic field, in repeating this cycle of applying the magnetic field and of measuring the optical density several times over the duration of the reaction between the analyte and the ligands, in calculating, by extrapolation, a limiting value for the optical density for an infinite magnetic field application time and in deducing therefrom the concentration of the analyte in the liquid medium.