Dissociation Between Dialysis Adequacy and Kt/V

Since the initiation of dialysis, nephrologists have sought an index (or indices) for the adequacy of toxic solute removal. This quest has been characterized by a gradual shift in thinking, ending with a preference for dynamic parameters such as clearances normalized for body size ( Kt / V ). The threshold Kt / V , however, has changed over the years. While present guidelines suggest 1.2 with single‐pool kinetics, higher levels might be proposed in the future. In spite of the known relation between Kt / V and survival, the accuracy of this parameter as a representative of the removal of the whole spectrum of compounds that are responsible for uremia is problematic. Kt / V only assesses the removal of a water‐soluble compound from the body water through mostly hydrophilic membranes to the dialysate water. Furthermore, the small size of urea means that convective and/or diffusive transfer through a given semipermeable membrane is unlikely to be representative of larger molecules, especially if dialyzers with a small pore size are applied. Urea kinetics are also poorly representative of the removal of small protein‐bound molecules and intracellular solutes with cell membrane‐limited clearance. Finally, it should be realized that the Kt / V concept has been developed in a specific population, that is, a group of renal failure patients with few comorbidities, submitted to short intermittent hemodialysis with small‐pore bioincompatible membranes very likely using dialysate of lower quality than that used today. Kt / V might well become less accurate and useful in predicting outcomes as different dialysis conditions are pursued, such as dialysis with biocompatible and/or large‐pore membranes, (ultra) pure dialysate, alternative time frames, high levels of convection, and/or in populations with a different distribution of body mass.