The present invention relates to an improved diagnostic device for analyte assay which has a cylindrical body portion and a removably attached cap. The cap has a liquid inlet extending into the cylindrical body portion and has a conical shaped portion leading to a liquid discharge aperture formed therein. A pre-filter may be formed either in the liquid inlet or in a pre-filter container which can be used with the device. The pre-filter container has a substantially cylindrical body which conforms to and can be inserted in the liquid inlet forming part of the removable cap. The container has an open upper end and an associated removable closure, a bottom end sealed with a frangible material and, if desired, a pre-filter positioned in the container between the upper and lower ends. When liquid is placed in the container, the container may be inserted in the liquid inlet and a puncture device associated with the liquid inlet ruptures the frangible material sealing the lower end and allows the liquid to be funneled to a reaction zone on a filter placed beneath or below the discharge aperture. Various size discharge apertures can be formed in the removable caps thereby allowing a particular cap to be associated with a device for a particular test.