Increases in mass transfer-area coefficients and urea Kt/V with increasing dialysate flow rate are greater for high-flux dialyzers.

The hemodialyzer mass transfer-area coefficient (K(o)A) for urea increases with increasing dialysate flow rate (Q(d)). The magnitude of the increase in K(o)A varies depending on the particular dialyzer under consideration; however, dialyzer properties that govern this phenomenon have not been established. We hypothesized that Q(d)-dependent increases in K(o)As are influenced by the water permeability of the dialysis membrane. We evaluated in vitro the effect of blood flow rate (Q(b)) and Q(d) on urea and creatinine K(o)As for two low-flux (Polyflux 6L and 8L) and two high-flux (Polyflux 14S and 17S) dialyzers containing Polyamide S membranes with similar membrane surface areas. Additional experiments were also performed on high-flux dialyzers containing Polyamide S membranes with very large surface areas (Polyflux 21S and 24S). K(o)As, calculated from the mean of blood- and dialysate-side clearances, were determined at zero net ultrafiltration for three different Q(b) and Q(d) combinations: Q(b) of 300 mL/min and Q(d) of 500 mL/min; Q(b) of 450 mL/min and Q(d) of 500 mL/min; and Q(b) of 450 mL/min and Q(d) of 800 mL/min. Urea and creatinine K(o)As were independent of the Q(b) but increased when Q(d) was increased from 500 to 800 mL/min. These increases in both urea and creatinine K(o)As were greater for high-flux than low-flux dialyzers (P < 0.0001). As expected, urea and creatinine K(o)As also increased with increasing membrane surface area. We conclude that dialysis membrane water permeability (or flux) is a dialyzer property that influences the dependence of small-solute K(o)As and clearance on Q(d). Whether this phenomenon is caused by enhanced internal filtration for dialyzers containing high-flux membranes requires further study.