Provided is a fluidic device including a main channel, wherein a first inlet fluidly connects to an upstream end of the main channel and introduces magnetic beads into a first side of the main channel. A second inlet is fluidly connected to the upstream end of the main channel and introduces a sample stream into a second side of the main channel. A magnet disposed adjacent to the second side of the main channel pulls the magnetic beads towards a sidewall of the second side, and thus into the sample stream. The beads continue through an extended incubation channel before entering a return channel. The return channel includes a detection region. Also provided is a multi-layer micro-fluidic assay device. An assay method that utilizes a microfluidic assay device is provided as well.