An arterial needle removes blood from the access to an extracorporeal circuit comprising a dialyzer and a venous needle returns the blood to the access site. Blood passes along one side of the membrane of the dialyzer and dialysis fluid along the other side. Fistula or access recirculation is detected by changing the blood flow rate, monitoring the concentration of a substance in the dialysate emitted from the dialyzer and correlating a change in the concentration to the change of the blood flow rate.
