An apparatus for extracorporeal blood treatment comprising: - at least one processing unit (3) having at least a first chamber (4) and at least a second chamber (5) separated from each other by a semipermeable membrane (6) - at least one blood collection line (7) connected to an inlet (4a) of the first chamber (4) and predisposed to draw blood from a patient (P) - at least one return line blood (8) connected to an outlet (4b) from the first chamber (4) and predisposed to return treated patient (P) blood, which line blood collection ( 7), the line - blood return (8) and the first chamber (4) are part of an extracorporeal blood circuit (2) at least one peristaltic pump (9) running in the extracorporeal blood circuit (2) to move the blood in the circuit (2) - at least one pressure sensor (13, 14) associated to the extracorporeal blood circuit (2) and configured to allow the determination of the pressure values in said extracorporeal blood circuit (2) - at least one drain line of blood (23) connected to an outlet of said second chamber (5) - a control unit (UC) connected to said at least one pressure sensor (13, 14) and the peristaltic pump (9) and configured to: - move the peristaltic pump (9) with a cyclic movement to generate a blood flow - receiving from said at least one (Ps) signal indicating the blood pressure in the extracorporeal blood circuit (2) pressure sensor (13, 14) - sampling, for each cycle of the peristaltic pump (9), (Ps) signal indicating the blood pressure with a frequency in the time domain depending on the speed (ω) of the peristaltic pump (9) wherein the control unit (UC) is also configured to: - receive a signal (ω, Fr) indicating the instantaneous blood flow (Qb) in the peristaltic pump (9) - comparing the signal (ω, Fr) with a plurality of fields (Δω1-Δωj, ΔFr1-ΔFrj) indicating predefined (ΔQb1-ΔQbj) blood flow each combined with a respective frequency of predefined sampling (Fs1-FSJ) intervals of (Ps) signal indicating the blood pressure -
