Hemodialyzer mass transfer-area coefficients for urea increase at high dialysate flow rates

The dialyzer mass transfer-area coefficient (K(o)A) for urea is an important determinant of urea removal during hemodialysis and is considered to be constant for a given dialyzer. We determined urea clearance for 22 different models of commercial hollow fiber dialyzers (N = similar to 5/model, total N = 107) in vitro at 37 degrees C for three countercurrent blood (Q(b)) and dialysate (Q(d)) flow rate combinations. A standard bicarbonate dialysis solution was used in both the blood and dialysate flow pathways, and clearances were calculated from urea concentrations in the input and ourput flows on both the blood and dialysate sides. Urea K(o)A values, calculated from the mean of the blood and dialysate side clearances, varied between 520 and 1230 ml/min depending on the dialyzer model, but the effect of blood and dialysate flow rate on urea K(o)A was similar for each. Urea K(o)A did not change (690 +/- 160 vs. 680 +/- 140 ml/min, P = NS) when Q(b) increased from 306 +/- 7 to 459 +/- 10 ml/min at a nominal Q(d) of 500 ml/min. When Q(d) increased from 504 +/- 6 to 819 +/- 8 ml/min at a nominal Q(b) of 450 ml/min, however, urea K(o)A increased (P < 0.001) by 14 +/- 7% (range 3 to 33%, depending on the dialyzer model) to 780 +/- 150 ml/min. These data demonstrate that increasing nominal Q(d) from 500 to 800 ml/min alters the mass transfer characteristics of hollow fiber hemodialyzers and results in a larger increase in urea clearance than predicted assuming a constant K(o)A.