Magnetic sensor device

The present invention provides a sensor device (20) and a method for determining the presence and/or amount of target moieties (22) in a sample fluid (27), the target moieties (22) being labeled with magnetic or magnetizable objects (23). The sensor device (20) comprises a magnetic field generating means adapted for applying a retention field for retaining the labeled target moieties (22, 23) to the sensor surface (24) during fluid manipulations. Through this the binding between the labeled target moieties (22, 23) and the sensor surface (24) and/or the magnetic particles (23) and the target moieties (22) is substantially not disturbed during these fluid manipulations. This leads to a reliable and accurate sensor device (20).