Device for continuous determination of a parameter (D, Cbin, K, Kt / v) significant of the efficacy of an extracorporeal blood treatment consisting of circulating on both sides of the semipermeable membrane (4) of an exchanger (1) membrane the blood of a patient and a treatment liquid, this device comprising: means (22, 26) for circulating in the exchanger (1) a treatment liquid having a characteristic (Cd) associated with the effectiveness of the treatment； means (16, 17, 19, 20； 19, 30) for causing a succession of variations of the characteristic (Cd) upstream of the exchanger (1)； means (30) for continuously memorizing a plurality of values (Cdin1 ... Cdinj ... Cdinp) of the characteristic (Cd) upstream of the exchanger (1)； means (25, 30) for continuously measuring and memorizing a plurality of values (Cdout1 ... Cdoutj ... Cdoutp) taken by the characteristic (Cd) downstream of the exchanger (1) in response to the variations of the characteristic ( Cd) caused upstream of the exchanger (1)； characterized by means (30) of calculation to calculate, each time a certain number of new values (Cdoutj) of the characteristic (Cd) downstream of the interc, air freshener (1), a parameter (D, Cbin, K, Significant Kt / v) of the effectiveness of extracorporeal blood treatment, from a first series of values (Cdinj) of the characteristic (Cd) upstream of the exchanger (1), of a second series of values (Cdoutj) of the characteristic (Cd) downstream of the exchanger (1), and by means of a mathematical model of the influence of the characteristic (Cd) on the treatment efficiency, the mathematical model having at least a coefficient consisting of a significant parameter (D, Cbin) of the effectiveness of extracorporeal blood treatment.Dispositivo de determinacion continua de un parametro (D, Cbin, K, Kt/v) significativo de la eficacia de un tratamiento extracorpOreo de la sangre que consiste en hacer circular a ambos lados de la membrana (4)semipermeable de un intercambiador (1) de
