Abstract: A diagnostic device comprises a container formed from a base element (10) and a cover element (11) bonded together to define a cavity. The cover element is formed with an aperture (17) surrounded by a depression (18) and a single or multi-layer porous support membrane (9) located within the cavity so as to be accessible through the aperture. A body of fluid-absorbent material (14) is also located within the cavity in contact with the porous support. The fluid-absorbent material (14) is formed at least partly from fibers of a kind which absorb fluid into the fiber itself, so as to cause the fiber to swell and increase in volume. Appropriate reactants are applied to the porous support (9) so that when a sample to be tested is applied to the support through the aperture (17) the result of any reaction can be observed. Samples or reactants in the form of fluids pass through the porous support and are absorbed by the underlying body (14) of fluid-absorbent material.