<p>The present invention relates to an apparatus for use in an assay comprising: (a) at least one first receptacle comprising a fluid inlet and a fluid outlet, (b) a porous membrane, and (c) at least one analyte-specific binding agent, characterised in that said at least one analyte-specific binding agent is immobilised on the underside of said porous membrane relative to the fluid inlet; and to methods of performing said assay.</p>
