A diagnostic device 1 for performing an immunochromatographic analysis of a sample is disclosed. The diagnostic device 1 comprises a porous substrate 10 for performing the analysis and a pre-filter 30 to remove solid components, such as solublised faeces or samples containing suspended organic material to prevent blocking of the porous substrate or coloured particles so that the test result is more clearly visible. The diagnostic device may include a container into which a sample may be provided, with the interior of the container being sealably connectable to the diagnostic section of the device. The porous section may be encapsulated within a housing.