This invention provides lateral flow filters with pore size gradients and with features to prevent peripheral flows around the filter. The filters can be laminated composites of two or more planar filter layers. Cartridges employing the filters can include a filtration chamber configured to retain the lateral flow filters including a port for sample application and a capillary channel for filtrate egress. The fluid egress port can be positioned to receive filtrate from one filter layer but not another.