A continuously repeatable method of detecting possible presence of one or several different selected target antigen(s), such as explosives and/or narcotics, in a predetermined test volume with a flow-through analysis device, such as a Piezoelectric Quartz Crystal Microbalance device (QCM), an Ellipsometer device or a Surface Plasmon Resonance biosensor device, is described. The device is equipped with one or several fluidly connected, individually operated, flow-through testing cells, each cell containing a sensing surface, each sensing surface comprising a selected modified target antigen immobilized thereon, which modified antigen has weaker affinity than the target antigen for a selected antibody specific for the selected target antigen. One testing cell is for analyzing a target antigen by the competition mode, whereas optional other testing cells are individually selected for analysis by the competition mode or the displacement mode.