Determining a presence of a target analyte in a fluid sample includes mixing multiple magnetic particles with the fluid sample, in which the magnetic particles are each bound to one or more binding moieties that specifically bind to the target analyte, flowing the fluid sample containing the magnetic particles through a fluidic channel, exposing the fluid sample in the fluidic channel to a magnetic field, measuring a signal from a Hall effect sensor while the fluid sample flows through the fluidic channel, and determining whether the target analyte is present in the fluid sample when the measured signal is in a first range of values.
