a differential conductivity recirculation monitor quantitatively determines the degree of recirculation in a fistula by comparing the conductivity of blood entering the fistula to the conductivity of blood being withdrawn from the fistula . a discrete quantity of a high conductivity marker fluid is injected into the blood entering the fistula , altering the conductivity of the blood entering the fistula . the altered conductivity blood enters the fistula and , if recirculation is present , co - mingles with blood in the fistula , altering the conductivity of the blood ion the fistula in proportion to the degree of recirculation . blood withdrawn from the fistula has an altered conductivity related to the degree of recirculation . quantitative values of the conductivity of the altered conductivity blood entering the fistula and the conductivity of the blood being withdrawn from the fistula are measured and a difference determined . the determined difference between the conductivities is used to determine a quantitative measurement of the degree of recirculation in the fistula .