A method for transferring a filter membrane (130) bearing a retentate to a reagent pad (310), in which the membrane is mounted in a support frame (120) having a shoulder (121) and a skirt (122) projecting from the shoulder, the pad being mounted on a cassette (300) of complementary form to said skirt, the skirt being adapted to slide sealingly on the cassette until it comes into abutment formed by the shoulder, the method comprising the steps of sealingly engaging the skirt of the membrane support frame on the cassette, establishing a pressure difference between the cassette and the membrane support frame such that the membrane adopts the shape of a dome pointing towards the pad, while maintaining the pressure difference, applying a force on the frame so as to make the skirt slide on the cassette until contact is established between the end of the dome and the pad, maintaining a force on the frame so as to move it downward until it comes into abutment formed by the shoulder while maintaining the pressure difference.