Abstract: An apparatus is provided for use in an immunoassay for the detection of a target analyte in a liquid sample, comprising a porous reactive filter, having an upper surface and a lower surface, to which is bound an antibody specific for the target antigen; a holder affixed to the porous filter, wherein the upper surface and lower surface of the reactive filter are accessible for passage of liquids through the reactive filter; an absorbent container capable of being detachably attached to the holder; and an absorbent material contained within the container so as to contact the lower surface of the porous filter when the absorbent container is attached to the holder. A prefilter assembly can optionally be attached to the holder so that the prefilter contacts the upper surface of the reactive filter.