A portable diagnostic device has a lysate stage with a port for receiving a sample and containing magnetic beads with a probe, and an outlet port. A series of assay stages are linked with the lysate vessel, each with a reservoir linked by channels. The final stage has a sensor for detecting beads attached to analyte molecules which have been conveyed according to attachment to probes on beads. Larger transport beads cause reporter beads which are tethered by target NA and probes to be transported to the final sensor stage, where they are released and detected when the transport beads have been removed.