The invention relates to a control unit (30) for detecting an overshoot of a first limit value (G1) of a first blood concentration (B1) in a first portion (17a) of a dialysate discharge line (17) downstream of a dialysate chamber (7) of a dialyser (4) of a blood treatment device and upstream of a node point (110) at which a bypass line (100) bypassing the dialyser (4) leads into the dialysate discharge line (17), wherein the bypass line (100) branches off, upstream of the dialysate chamber (7), from a dialysate supply line (15) suitable for supplying dialysate from a dialysate source (16) to the dialysate chamber (7).