A device with a selectively permeable membrane intended to be used to isolate the cellular components of a cytology specimen suspended in a transport or culture medium, for subsequent in vitro diagnostic investigation. It is typically in the form of a mesh-like film which may be encased in plastic, designed to fit onto the opening of the cytology specimen container, to capture and collect the cells while allowing the medium to flow from the receptable through the membrane. This is a single-use device.