The present invention relates to an apparatus and method for monitoring access to the patient's extracorporeal blood treatment device comprising an extracorporeal blood circulation circuit. Furthermore, the present invention relates to extracorporeal blood treatment device comprising a device for monitoring the vascular access. Method according to device and the present invention according to the present invention, specific properties of the blood, in particular, is based on the monitoring of the concentration of hemoglobin in the blood flowing in the arterial blood conduit of the extracorporeal blood circulation I of the extracorporeal blood treatment device the A. At the time of the improper vascular access will change the flow of the situation in the contact of the body and the extracorporeal blood circulation system. These changes in the flow of the situation, can be detected as a change in hemoglobin concentration. Missing out venipuncture cannula 8 for access to the patient is confirmed by a reduction in the hemoglobin concentration of the blood of the arterial blood conduit 6.
.FIELD 1