An apparatus for extracorporeal blood treatment comprising: - at least a treatment unit having at least a first chamber and at least a second chamber separated from one another by a semi-permeable membrane; - at least a blood removal line connected to an inlet port of the first chamber and predisposed to remove blood from a patient; - at least a blood return line connected to an outlet port from the first chamber and predisposed to return treated blood to the patient, wherein the blood removal line, the blood return line and the first chamber are part of an extracorporeal blood circuit; - at least a peristaltic pump operating at the extracorporeal blood circuit for moving the blood in the circuit; - at least a pressure sensor (13, 14) associated to the extracorporeal blood circuit and configured to enable determination of pressure values in said extracorporeal blood circuit; - at least a fluid drainage line connected to an outlet port of said second chamber; - a control unit connected to said at least a pressure sensor (13, 14) and to the peristaltic pump and configured to: - move the peristaltic pump with a cyclic motion to generate a blood flow; - receive from said at least a pressure sensor (13, 14) a signal (Ps) indicating the pressure of the blood in the extracorporeal blood circuit; - sample, for each cycle of the peristaltic pump, the signal (Ps) indicating the blood pressure with a frequency in the time domain depending on the velocity (ω) of the peristaltic pump; characterized in that the control unit is further configured to: - receive a signal (ω, Fr) indicating the instant blood flow (Qb) in the peristaltic pump; - compare the signal (ω, Fr) with a plurality of fields (Δω 1 -Δω j, ΔFr 1 -ΔFr j) indicating predefined intervals of blood flow (ΔQb 1 -ΔQb j) each combined with a respective predefined sampling frequency (Fs 1 -Fs j) of the signal (Ps) indicating the blood pressure; - identify the field (Δω k, ΔFs k) in the plurality of fields (Δω 1 -Δω j, ΔFr 1 -ΔFr j) to which the signal belongs (ω, Fr); - identify the respective predefined sampling frequency (Fs k); - dimension a filter to be used on the basis of the predefined sampling frequency (Fs k) and at a desired cutting frequency (Fc); - apply the filter to the sampled signal (Ps) indicative of the blood pressure. - at least a treatment unit having at least a first chamber and at least a second chamber separated from one another by a semi-permeable membrane; - at least a blood removal line connected to an inlet port of the first chamber and predisposed to remove blood from a patient; - at least a blood return line connected to an outlet port from the first chamber and predisposed to return treated blood to the patient, wherein the blood removal line, the blood return line and the first chamber are part of an extracorporeal blood circuit; - at least a peristaltic pump operating at the extracorporeal blood circuit for moving the blood in the circuit; - at least a pressure sensor (13, 14) associated to the extracorporeal blood circuit and configured to enable determination of pressure values in said extracorporeal blood circuit; - at least a fluid drainage line connected to an outlet port of said second chamber; - a control unit connected to said at least a pressure sensor (13, 14) and to the peristaltic pump and configured to: - move the peristaltic pump with a cyclic motion to generate a blood flow; - receive from said at least a pressure sensor (13, 14) a signal (Ps) indicating the pressure of the blood in the extracorporeal blood circuit; - sample, for each cycle of the peristaltic pump, the signal (Ps) indicating the blood pressure with a frequency in the time domain depending on the velocity (ω) of the peristaltic pump; - move the peristaltic pump with a cyclic motion to generate a blood flow; - receive from said at least a pressure sensor (13, 14) a signal (Ps) indicating the pressure of the blood in the extracorporeal blood circuit; - sample, for each cycle of the peristaltic pump, the signal (Ps) indicating the blood pressure with a frequency in the time domain depending on the velocity (ω) of the peristaltic pump; - receive a signal (ω, Fr) indicating the instant blood flow (Qb) in the peristaltic pump; - compare the signal (ω, Fr) with a plurality of fields (Δω 1 -Δω j, ΔFr 1 -ΔFr j) indicating predefined intervals of blood flow (ΔQb 1 -ΔQb j) each combined with a respective predefined sampling frequency (Fs 1 -Fs j) of the signal (Ps) indicating the blood pressure; - identify the field (Δω k, ΔFs k) in the plurality of fields (Δω 1 -Δω j, ΔFr 1 -ΔFr j) to which the signal belongs (ω, Fr); - identify the respective predefined sampling frequency (Fs k); - dimension a filter to be used on the basis of the predefined sampling frequency (Fs k) and at a desired cutting frequency (Fc); - apply the filter to the sampled signal (Ps) indicative of the blood pressure.