Once upon a time in dialysis: the last days of Kt/V?

After its proposal as a marker of dialysis adequacy in the eighties of last century, Kt/V(urea) helped to improve dialysis efficiency and to standardize the procedure. However, the concept was developed when dialysis was almost uniformly short and was applied thrice weekly with small pore cellulosic dialyzers. Since then dialysis evolved in the direction of many strategic alternatives, such as extended or daily dialysis, large pore high-flux dialysis, and convective strategies. Although still a useful baseline marker, Kt/V(urea) no longer properly covers up for most of these modifications so that urea kinetics are hardly if at all representative for those of other solutes with a deleterious effect on morbidity and mortality of uremic patients. This is corroborated in several clinical studies showing a dissociation between removal of urea and that of other uremic toxins. In addition, randomized controlled trials showed no benefit of increasing Kt/V(urea). Finally, this parameter also hardly is evocative for metabolic or intestinal generation of toxins, for their removal by residual renal function and for the complex interaction of dialysis length with removal pattern and patient outcomes. We conclude that apart from being a baseline parameter of dialysis adequacy, Kt/V(urea) insufficiently represents all novel strategic changes of modern dialysis. Kt/V(urea) is too simple a concept for the complexities of uremia and of today's dialysis.