This invention provides a method and apparatus for calculating a hemodialysis parameter, especially blood access flow rate, using multiple dialysance values. The dialysance values may be calculated based upon either sodium or urea concentrations. One dialysance value can be determined for conditions in which a patients arterial line withdraws blood from an upstream location in a patients fistula and treated blood is returned by a venous line to a downstream location in a patients fistula. The second dialysance value can be determined when the lines have been reconfigured so that the arterial line withdraws blood from a downstream portion of a patients fistula and the venous line returns treated blood to an upstream portion of a patients fistula. Since it is possible to determine the dialysance values solely from concentration measurements made on the dialysate side of the dialysis apparatus, the present method and apparatus provide a non-invasive means for determining hemodialysis parameters such as blood access flow rate and recirculation.
