covalent immobilization of the antigen onto carboxylic-modified magnetic beads (HOOC-MBs) activated with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysulfosuccinimide (sulfo-NHS), and further incubation in a mixture solution containing variable concentrations of the antigen and a fixed concentration of an HRP-labeled detection antibody.