Abstract: In an extracorporeal blood treatment system, non-treated blood may be withdrawn from a patient through an arterial line connected to a first compartment of a blood treatment apparatus. The blood treatment apparatus also includes a second compartment separated from the first component by a semi-permeable membrane. Treated blood may be returned to the patient through a venous line connected to the second compartment. In order to determine blood recirculation, i.e., a mixture of treated blood and non-treated blood in the arterial line, the ultrafiltration rate of plasma water through the semi permeable membrane of the treatment apparatus may be temporarily increased or decreased, and the resulting variation of the blood concentration, i.e., of the ratio of the volume of the plasma volume to the blood volume, is determined.

Abstract: A method for determining the recirculation of blood in a vascular access (11) of a patient whose blood is subjected to a treatment in a blood treating apparatus (12) connected to the patient by an extra-corporal circuit comprises the following steps: regulating the flow rate of in the extra-corporal circuit at a predetermined value (QB); determining a first value (HGBA0) of the parameter representing the concentration of blood in the extra-corporal circuit; bringing about a first variation of predetermined amplitude (QUF) of the ultrafiltration flow rate in the treating apparatus (12); determining a second value (HGBA3) of the parameter representing the concentration of blood in the extra-corporal circuit; computing the recirculation (AR) of the blood in the extra-corporal circuit on the basis of the blood flow rate (QB), of the predetermined amplitude (QUF) of the first variation of the ultrafiltration flow rate and of the first and second values (HGBA0, HGBA3) of the parameter representing the blood concent