The present invention relates to a device for creating a vascular access for extracorporeal blood treatment including an electrically conductive puncture needle with a proximal endpiece, a distal endpiece, and a base body made of electrically non-conductive material into which the distal endpiece is inserted. In a properly arranged vascular access, the base body, including an electrically conductive contact element, lies on the skin of the patient, when the puncture needle is located within the vessel of the patient. A disconnection or dislocation is detected by the impedance between the electrically conductive puncture needle and the electrically conductive contact element being measured and compared to a limit value. If the impedance measured is within predetermined limits, this indicates a properly arranged vascular access. A disconnection or dislocation of the puncture needle leads to a strong variation in the impedance.