The invention relates to a device and a method for detecting recirculation for an extracorporeal blood treatment device, and to an extracorporeal blood treatment device having a device for detecting recirculation. The device according to the invention and the method according to the invention are based on a targeted haemodilution by administering a bolus of a substitution fluid (substituate), which leads to a change in the pressure conditions in the venous and arterial branches (6, 7) of the extracorporeal circuit as a result of a change in the viscosity of the fluid flowing in the extracorporeal blood circuit (5A). Recirculation is determined by detecting the pressure change in the extracorporeal circuit. The device for detecting recirculation has a control unit (17), which cooperates with the blood conveying device (8) in the blood treatment device and with the substituate conveying device (16) in the blood treatment device. The control unit (17) provides an operating mode for detecting recirculation, in which the blood flow rate is reduced during administration of a substituate bolus. With the simultaneous bolus administration of substituate and reduction of the blood flow rate, the composition of the fluid flowing through the extracorporeal circuit is optimized for detecting recirculation, and therefore larger pressure changes in the venous and arterial branches of the extracorporeal circuit are produced. The overall sensitivity and reliability of the measuring method is thereby improved.
