An apparatus for treating a medical fluid is provided which includes a treatment machine having a coupling surface, with a cassette made of a hard part having liquid-conducting passages which are covered by a flexible film being able to be coupled to the coupling surface of the treatment machine. A corresponding medical cassette is also provided. An air-free coupling of the flexible film of the cassette to the treatment machine is ensured in that air can be sucked out areally along a plane between the flexible film and the coupling surface during the coupling process and/or with a coupled cassette. A method for coupling the cassette and a method for checking the leaktightness of the cassette via the vacuum control of the air suction is also provided.
