Disclosed are devices and methods for performing an assay which determines the presence or quantity of an analyte in a fluid sample by detecting binding of the analyte to at least one immobilized analyte capture reagent and washing unbound material from the immobilized analyte capture reagent. The devices and methods an elongated solid phase flow matrix, including capillary channels capable of driving capillary fluid movement, and further including (i) a first region adapted for receipt of the fluid sample, (ii) a second region at which the analyte capture reagent is immobilized, (iii) a third region for application of a wash reagent capable of removing unbound substances from the second region; and (iv) an absorbent reservoir of high volume capacity. The second region is positioned intermediate to the first region and the third region and intermediate to the absorbent reservoir and the third region. The device also includes means to detect analyte bound at the second region. The flow matrix and the regions thereof are sized and positioned to cause the fluid sample to flow initially along the elongated flow matrix in one direction toward and through the second region, and subsequently, the liquid reagent to flow along the elongated flow matrix in a second direction opposite the first direction, through the second region, and into the absorbent reservoir, drawing unbound substances with it.
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