The invention relates to a method for checking a connection between a compressed air outlet (1001) of a blood treatment apparatus (1000) and a pressure measuring line (105) of an extracorporeal blood tube set (100), wherein the method comprises the steps: providing a blood treatment apparatus (1000) which has a compressed air line (1005)； a compressed air device (1003) in fluid communication with the compressed air line (1005) for generating pressure within the compressed air line (1005)； a compressed air outlet (1001) in fluid communication with both a compressed air line (1005) and an exterior of the blood treatment apparatus (1000), wherein the compressed air outlet (1001) is designed in order to be connectable to a pressure measuring line (105)； and a pressure sensor (1007) which is arranged for measuring the prevailing pressure (P) at the compressed air outlet (1001) or in the compressed air line (1005)； building up of a pressure and an air flow in the compressed air line (1005) and/or at the compressed air outlet (1001) by means of the compressed air device (1003)； measuring of a prevailing pressure (P) at the compressed air outlet (1001) or in the pressure line (1005), or of a pressure change over time by means of the pressure sensor (1007)； and evaluating of a rise in the measured pressure (P) or of a rise in the measured pressure over time, respectively by means of a comparison of the measured pressure (P) or a measured pressure rise with the previously measured or stored values, threshold values (P-V), regions or profiles.
