Abstract: A method and a device for performing an assay in order to detect or determine the amount of an analyte in a test liquid, wherein bound and unbound reactants can be separated, comprising a capillary canal for liquid transport that is at least partly bordered by a semi-permeable layer, wherein during performance of an assay a movable solid phase material bearing a ligand capable of binding the analyte or binding a reactant for the analyte is within the capillary channel adjacent to the semi-permeable layer, said semi-permeable layer having pores that are sufficiently small to prevent the passage of the movable solid phase material and sufficiently large to permit passage of unbound reactants there through.