Method for determining the transmembrane pressure during extracorporeal blood treatment, where the blood flows with a given rate of blood flow through a conduit arterial blood supply (6) of an extracorporeal blood circuit (5A) towards the inlet (3a ) a first chamber (3) of a dialyser (1) divided into first and second chamber (3, 4) through a semipermeable membrane (2), and flows through a return duct venous blood (7) from the outlet (3b) of the first chamber (3) of the dialyser (1), and dialysis liquid circulates through a supply conduit dialysis fluid (10) to the inlet (4a) of the second chamber (4) dialyser (1), and flows through a discharge line dialysis fluid (11) from the outlet (4b) of the second chamber (4) of the dialyser (1), where the blood with a flow rate determined is draws liquid through the membrane (2) of the dialyser (1), with l os following method steps: measuring the pressure of the blood side on the inlet or outlet of the first chamber of the dialyser and on the side of the dialysis liquid in the inlet or outlet of the second chamber of the dialyser, and calculating of the transmembrane pressure based on the pressure measured in the blood side and on the side of the dialysis liquid, characterized in that, a variable that is correlated with blood viscosity is determined, in that a correction variable is determined for the transmembrane pressure, which depends on a variable that is correlated with blood viscosity, where to determine the correction variable for the transmembrane pressure characteristic parameter of the flow resistance of the dialyzer is determined, which depends parameter that is correlated with blood viscosity, and that the transmembrane pressure is calculated based on the measured pressure in the blood side and on the side of the dialysis liquid and the correction variable of the transmembrane pressure.Método para determinar la presión de transmembrana durante un tratamiento de sangre extracorporal, donde la sangre circula con una
