An assay device (1) for determining the presence and/or amount of an analyte present or potentially present in a liquid sample comprises: (i) a capillary tube (2) having an upstream region (3) into which the sample to be assayed is introduced for transfer by capillary action along the capillary tube to a downstream region thereof; (ii) a collection of first binding partners (5) immobilized within the capillary tube (2), said first binding partners (5) being capable of specifically binding to the analyte; (iii) a collection of second binding partners (6) displaceabley bound to a fraction of said first binding partners (5) whereby there are free first binding partners (5) immobilized within the capillary tube, said second binding partners (6) having a label and being displaceable from the first binding partners (5) by the analyte to be detected; and (iv) a detection region (4) for sample that has transferred to said downstream region of said capillary tube, said detection region being adapted to generate a detectable signal from the label on displaced second binding partners (6) that have transferred to the downstream region.
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