Analytical assay device and methods using surfactant treated membranes to increase assay sensitivity

An analytical device for use in the detection of a target substance in a liquid sample, said device comprising a porous flow-through reaction membrane having an exposed sample-contacting surface and at least one receptor area located in a limited region of said exposed sample-contacting surface, wherein said reaction membrane contains at least one surfactant having a higher concentration within said limited region relative to the area peripheral to said limited region, and wherein said limited region of said reaction membrane has been pre-treated with a surfactant-containing solution comprising at least about 0.2% surfactant that results in said higher concentration of said surfactant within said limited region, and an absorbent body in fluid communication with the opposite side of said reaction membrane from said sample-contacting surface, whereby liquid sample deposited on said exposed sample-containing surface flows through said membrane to said absorbent body, wherein said surfactant in said limited region permits the liquid sample to flow through said limited region at a faster rate than the liquid sample flowing through said peripheral area.