Effect of Low Dialysate Flow Rate on Hemodialyzer Mass Transfer Area Coefficients for Urea and Creatinine

Recent work has shown that the dialyzer mass transfer area coefficient (K_o A) for urea increases when the dialysate flow rate is increased from 500 to 800 mL/min. In this study we determined urea and creatinine clearances for two commercial dialyzers containing polysulfone hollow fibers in vitro at 37°C, a nominal blood flow rate of 300 mL/ min, and dialysate flow rates (Q_d ) ranging from 100 to 800 mL/min. A standard bicarbonate dialysis solution was used in both the blood and dialysate flow pathways, and clearances were calculated from solute concentrations in the input and output flows on both the blood and dialysate sides. Urea and creatinine K_o A values, calculated from the mean of the blood and dialysate side clearances, increased (p < 0.01) with increasing Q_d over the entire range studied. The increase in both urea and creatinine K_o A with increasing Q_d was proportional to the K_o A value. These data show that changes in Q_d alter small solute clearances greater than predicted assuming a constant K_o A.