A method and apparatus for use in a flow through assay process is disclosed. The method is characterised by a "pre-incubation step" in which the sample which is to be analysed, (typically for the presence of a particular protein), and a detection analyte (typically an antibody bound to colloidal gold or a fluorescent tag) which is known to bind to the particular protein may bind together for a desired period of time. This pre incubation step occurs before the mixture of sample and detection analyte come into contact with a capture analyte bound to a membrane. The provision of the pre-incubation step has the effect of both improving the sensitivity of the assay and reducing the volume of sample required for an assay. An apparatus for carrying out the method is disclosed defining a pre-incubation chamber for receiving the sample and detection analyte having a base defined by a membrane and a second membrane to which a capture analyte is bound. In one version the pre-incubation chamber is supported above the second membrane in one position but can be pushed into contact with the membrane carrying the capture analyte thus permitting fluid transfer from the incubation chamber through the capture membrane. In another version the membrane at the base of the incubation chamber is hydrophobic and its underside contacts the capture membrane and when a wetting agent is applied to the contents of the pre-incubation chamber fluid transfer occurs.