Method for detecting and quantifying an analyte in a liquid sample, using a test strip and magnetic particles as a detectable label and based on the detection of an amount of magnetic particles which become bound to a reading zone of a test strip as a result of performing the method, said amount being linked through a function to the analyte content of the sample, wherein the magnetic particles exhibit a nonlinear magnetization characteristic, which test strip is made of porous material and is arranged inside a case, cartridge or the like to form an assay device, wherein in its part surrounding the reading zone, the case is made of a material that is permeable to magnetic field, wherein for the reading of the reaction, the assay device is positioned in a measure cell of a magnetic reading device which detects the amount of magnetic particles in the reading zone by submitting it to at least one excitation magnetic field having about 90% of its power within one frequency band or a plurality of frequency bands, and the reading device measures the induced magnetic response field outside said frequency band or said plurality of frequency bands.