A machine for extracorporeal blood treatment, comprising: a blood treatment device (4) having a blood chamber (5) and a fluid chamber (6), separated from each other by the semipermeable membrane (3), an extracorporeal blood circuit comprising an arterial route (7), to carry blood withdrawn from a patient to an entrance of the blood chamber (5), and a venous route (8), to return treated blood from an outlet of the blood chamber (5) to the patient. a device for ultrafiltration (2) of a liquid through the semipermeable membrane (3) of the blood treatment device (4)； a controller (14) programmed to perform the following operations: change an operating situation of said ultrafiltration device (2)； detecting a plurality of values of a first quantity indicating an operational situation of said ultrafiltration device (2) when changing said operative situation； evaluating an optimum value of said first quantity from a comparison of said values, the operation of evaluating an optimal value of said first quantity comprises the phases of: determining at least one change in said first quantity caused by said change in said operating situation ； determining at least one change in a second quantity indicating an operating situation of said ultrafiltration device caused by said change in said operating situation； checking whether said change in said first quantity and said change in said second quantity match a predetermined relationship with a reference value； generate a signal consistent with said verification phase； wherein said first quantity comprises an ultrafiltration flow rate through said membrane (3), or an amount on which said ultrafiltration flow rate depends, and wherein said second quantity is a transmembrane pressure of said semipermeable membrane (3).Una máquina para tratamiento sanguíneo extracorpóreo, que comprende: un dispositivo de tratamiento sanguíneo (4) que tiene una cámara sanguínea (5) y una cámara de fluido (6), separadas entre sí por la membrana semiperm
