Normal milk samples are opalescent and transparent, and stratified ones cannot be used. The main ingredients of milk are water, fat, lactose and salt. The sample contains certain protein-precipitated substances, and the presence of fat will affect the detection result. For the sample preparation, it is recommended to centrifuge to remove the insoluble matter and the upper layer of fat, and add a certain buffer solution, vortex and mix well to keep the sample’s stability in the immunoassay reaction. Or sterilized and inactivated, placed in frozen conditions for long-term storage.