A sample loading cartridge (1) for a microfluidic device comprises a cartridge body (10) with a sample reservoir (20) configured to house a volume of a liquid sample (3) and a sample port (30) in connection with the sample reservoir (20). The cartridge (1) also comprises an output channel (40) extending from the sample reservoir (20) and a feedback channel (50) connected to the sample reservoir (20) and to the sample port (30). The cartridge body (10) comprises a detection portion (60) aligned with the feedback channel (50) to enable detection of any sample (3) in the feedback channel (50). The flow resistance of the feedback channel (50) is lower than the flow resistance of the output channel (40) to cause liquid sample (3) received in the sample port (30) to enter the feedback channel (50) with substantially no liquid sample (3) entering the output channel (40).