A device for detecting at least one substance of interest in a liquid sample, the device comprising: (a) a unit comprising a porous material (210, 910) for receiving the liquid sample, wherein the porous material (210, 910) is capable of absorbing and transmitting at least a portion of the liquid sample, (b) a porous membrane (220, 920) that comprises a specific binding pair member that is specific for the substance of interest or a substance bound to the substance of interest, wherein the specific binding pair member is an antibody, the porous material being larger than the area of the porous membrane to provide a liquid sample application zone comprising a sample application pad, the porous material being configured for allowing diffusion of the liquid sample between the porous material and the porous membrane, and (c) a wash solution receiving pad (840, 940), wherein the unit and porous membrane are each shaped to permit the porous membrane to be in direct contact with the porous material over at least a portion of the porous membrane that comprises the specific binding pair member, wherein the porous material is in physical contact with a bottom side of the porous membrane comprising the specific binding pair member, the specific binding pair member being located above the porous material, the porous membrane being in direct contact with the porous material at a site where the specific binding pair member is located, the device further comprising a container containing the unit, the container comprising a sample application port and a viewing window separate from the sample application port, the sample application port and the viewing window being located on a same side of the container, the sample application port permitting application of the liquid sample to the sample application pad, the device being configured to allow detection of the substance bound to the specific binding pair member by viewing the porous membrane through the detection window, and wherein the container exerts pressure on the porous material from below with respect to the porous membrane at the site of the porous membrane to cause compression of the porous material at this site and at a reaction site of the porous material, and to ensure complete contact of the porous membrane and the porous material over at least a portion of the area where the specific binding pair member is located, and wherein the porous material comprises a reaction pad that is compressed as compared to the sample application pad and the wash solution receiving pad. (a) a unit comprising a porous material (210, 910) for receiving the liquid sample, wherein the porous material (210, 910) is capable of absorbing and transmitting at least a portion of the liquid sample, (b) a porous membrane (220, 920) that comprises a specific binding pair member that is specific for the substance of interest or a substance bound to the substance of interest, wherein the specific binding pair member is an antibody, the porous material being larger than the area of the porous membrane to provide a liquid sample application zone comprising a sample application pad, the porous material being configured for allowing diffusion of the liquid sample between the porous material and the porous membrane, and (c) a wash solution receiving pad (840, 940), wherein the unit and porous membrane are each shaped to permit the porous membrane to be in direct contact with the porous material over at least a portion of the porous membrane that comprises the specific binding pair member, wherein the porous material is in physical contact with a bottom side of the porous membrane comprising the specific binding pair member, the specific binding pair member being located above the porous material, the porous membrane being in direct contact with the porous material at a site where the specific binding pair member is located, the device further comprising a container containing the unit, the container comprising a sample application port and a viewing window separate from the sample application port, the sample application port and the viewing window being located on a same side of the container, the sample application port permitting application of the liquid sample to the sample application pad, the device being configured to allow detection of the substance bound to the specific binding pair member by viewing the porous membrane through the detection window, and wherein the container exerts pressure on the porous material from below with respect to the porous membrane at the site of the porous membrane to cause compression of the porous material at this site and at a reaction site of the porous material, and to ensure complete contact of the porous membrane and the porous material over at least a portion of the area where the specific binding pair member is located, and wherein the porous material comprises a reaction pad that is compressed as compared to the sample application pad and the wash solution receiving pad.