Assay device for a suspension

An assay device configured to separate a suspension into a filtrate and a residue for testing at least one of the filtrate and the residue, e.g. for testing for sepsis. The device comprises a filter aperture 410 for a filter membrane for separating the suspension into the filtrate and residue. The device further comprises a filtrate receiving portion 510; a residue receiving portion 520; a first buffer container (350, fig 3); a liquid release member 420 configured to create an outlet of the first buffer container; a filter arm 400 comprising the filter aperture; and a user actuated switch (110, fig 1) comprising a transition structure 200 for guiding the filter arm. The device is configured such that at the first set position the filter aperture is aligned with the filtrate receiving portion, and at the second set position the filter aperture is aligned with the first buffer container and with the residue receiving portion. During a transition from a first to a second position of the switch, the filter arm moves the filter aperture into alignment with the residue receiving portion; such that the liquid release member creates the outlet of the first buffer container.