The ultrafiltration coefficient of a dialyser (KUF) is not a fixed value, and it follows a parabolic function: the new concept of KUF max

Background. Hydraulic permeability (KUF) is an intrinsic characteristic of dialysers, reported by the manufacturer as a single value, which drives and limits fluid removal. High-flux dialysers have been introduced with the appearance of convective techniques, aiming to increase fluid and solute removal. High convective volumes are being employed, although their advantages have not been fully demonstrated. Methods. We assessed KUF over a pre-selected range of ultrafiltration rates (QUF) in post-dilutional haemodiafiltration and high-flux haemodialysis. Results. KUF vs QUF was neither a fixed value nor a linear function but followed a parabolic function with a vertex der (y) = 0, which we have called KUF max. This also held true in high-flux routine dialysis. Conclusions. These findings are completely new and have clear applications in clinics. The vertex point might be used to define the optimal QUF of a dialysis system, which would be that obtained at KUF max and corresponds to the best QUF/transmembrane pressure ratio, as opposed to the maximum QUF (which corresponds to the highest possible QUF), frequently associated with haemoconcentration, clotting, loss in dialyser surface area, and treatment problems. Determining KUF max in vivo could be of help in dialysis prescription and control with automatic systems.