that is higher or lower than the fluid flowing in the outlet line 32 in the preferred embodiment a high conductivity marker fluid is used to avoid damaging blood cells in the preferred embodiment the bolus is 1 milliliter of 24 percent hypertonic saline solution the conductivity of the treated blood being returned to the patient through the dialyzer outlet line 32 and the outlet conductivity cell 52 of the recirculation monitor 22 is altered this altered conductivity blood enters the fistula through the outlet needle 34 if the flow balance in the fistula 26 is such that no flow is recirculating the altered conductivity blood will exit the fistula as illustrated by the flow circulation arrow 94 without altering the conductivity of the blood within the fistula if however the flow balance within the fistula 26 is such that blood is recirculating as illustrated by flow circulation arrow 96 a portion of the blood withdrawn from the fistula 26 by the pump 36 will be the altered conductivity blood the recirculation monitor 22 measures the conductivity of the blood flowing in the outlet line 32 and the conductivity of the blood flowing in the inlet line 30 and quantitatively determines the difference between those conductivities continuously throughout the recirculation test the sensing logic and display circuit 90 interprets the quantitative conductivity differences measured by the recirculation monitor 22 to determine recirculation efficiency the determination of recirculation efficiency will be explained by reference to fig4 and 6 the outlet conductivity cell 52 and the inlet conductivity cell 54 may be thought of as signal generators generating the induced currents 80 and.