disclosed are assay systems , kits , 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 . an elongated solid phase flow matrix includes : a first region adapted for receipt of the fluid sample , a second region at which the analyte capture reagent is immobilized , a third region for application of a liquid wash and / or detector reagent capable of removing unbound substances from the second region ; and an absorbent reservoir of high volume capacity . the second region is positioned between the first region and the third region and between the absorbent reservoir and the third region . the device also includes means to detect analyte bound at the second region . the fluid sample flows by capillary action initially along the elongated flow matrix in one direction toward and through the second region , and subsequently , liquid wash and / or detector reagent flows along the matrix in a second direction opposite the first direction , through the second region , and into the absorbent reservoir , drawing unbound substances with it .