Abstract: An apparatus for conducting an assay having an agglutination or size separation step, includes: a first section disposed to receive a fluid to be assayed; and a second section disposed to receive the fluid from the first section upon application of a motive force, preferably centrifugal force, to the fluid, the second section comprising elements fixed to a substrate and adapted to mix the fluid and trap agglutinated particles. In a preferred embodiment, the elements are shaped as pillars. In another preferred embodiment, a third section is provided after the second section and the apparatus is in the form of a disk, preferably an optical disk, having a central axis, and wherein the first, second and third section are arranged in the disk as channels in a direction away from the central axis, respectively.