The invention relates to a device for treating a medical fluid, said device comprisinga treatment machine with a coupling surface, to which a cassette consisting ofa hard part with fluid-conducting channels that are covered by a flexible filmcan be coupled. The invention also relates to a corresponding medical cassette.According to the invention, to guarantee that the film of the cassette can be coupledto the treatment machine without the inclusion of air, air can be sucked off alongthe plane lying between the flexible film and the coupling surface during thecoupling process and/or when the cassette is coupled to the machine. The inventionalso relates to a method for coupling the cassette and to a method for checkingthe leak-tightness of the cassette by means of the regulation of the air removalby suction.
