A lateral-flow assay device includes a substrate with a sample addition zone and a fluid flow path. A cover supports a filter having a portion contacting the substrate to create a contact area that at least partly overlaps the sample addition zone. Another portion extends from the contact portion to the supported periphery of the filter to define with the substrate a reservoir configured to retain the filtrate by capillary pressure between the substrate and the extending portion. The reservoir volume is based on an acute angle formed between the substrate and the extending portion of the filter and on a fluid meniscus of the filtrate. The filter and sample addition zone are configured to provide capillary pressure drawing the filtrate from the reservoir to the sample addition zone. Methods for controlling flow characteristics in a lateral-flow assay device are also described.
