A control device for a blood treatment machine performs a connection test (50) by causing the blood treatment machine to switch (51, 53) between a first and a second operating state by reversing a blood pump so as to change a flow direction of blood through both a dialyzer and access devices connected to a patient. Based on an output signal of at least one sensor in the blood treatment machine (52, 54), the control device computes (55) an efficiency change parameter that represents a change in in-vivo clearance of the blood treatment machine during the switch of the blood treatment machine between the first and second operating states. The control device evaluates (56) the efficiency change parameter to jointly detect connection errors at the dialyzer, resulting in co-current flow of treatment fluid and blood through the dialyzer, and at the access devices, resulting in access recirculation of blood.
