In addition to concerns about the accuracy of the displayed blood flow rate, the effective blood flow rate may be lower than the pumped blood flow rate because of access recirculation. Access recirculation occurs when the blood flow entering the access from the systemic circulation is insufficient to provide the chosen extracorporeal blood flow rate. When that occurs, some of blood returning to the acces from the dialyzer is immediately recycled back to the dialyzer. Traditionally, access recirculation was determined from blood urea concentrations in the extracorporeal circuit and systemic circulation. This methode have several drawbacks, it may be confounded by cardiopulmonary recirculation and it may require several days to obtain the necessary urea concentrations from a clinical laboratory. An alternative approach to monitoring access performance is to measure the access blood flow rate. Access blood flow rate can be measured using indicator dilution methods. This technique provides a reliable estimate of access blood flow rate but requires additional equipment. Access blood flow rate can also be calculated from clearances measured with the blood lines in the normal configuration and in a reverse configuration. This method has the advantage that several dialysis machines now allow the automated non-invasive measurements of ionic clearance based on dialysate conductivity measurements.