Efficiency of a dialyzer is defined by its urea KoA, the maximum urea removal capability of the dialyzer.
Leypoldt, J. & Cheung, A. (2001). Increases in mass transfer-area coefficients and ureas Kt/V with increasing dialysate flow rate are greater for high-flux dialyzers. American Journal of Kidney Diseases, 38 (3), pp. 575-579.