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

Abstract

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.

Urea, a true uremic toxin: the empire strikes back

“…Haemodialysis therapy can itself exert adverse effects including activation of inflammatory pathways through blood exposure to the extracorporeal circuit as well as indiscriminate removal of useful small molecule nutrients. Finally, the ESRD milieu is extremely complex whereby variations in residual kidney function, middle molecule clearance, non-urea solute fluctuations, mineral/bone abnormalities and whole-body fluid balance all impact patient outcomes, leading some experts to denounce urea clearance (Kt/V) as an over-simplified measure of dialysis adequacy [17]. Indeed, efforts to investigate the potential benefits of urealowering may be most productive if focused on the pre-dialysis population, as with low protein diets discussed below.…”

Kt/V: A Magical Formula for Dialysis Adequacy: A Critical Review

“…In other words, the dialysis adequacy algorithm includes subjective and objective aspects, and an efficient dialysis leads to a subjective feeling of well-being (29).…”

“…Thus, urea production and removal alone cannot exhibit all the spectrum of uremic toxicities (28). Furthermore, urea kinetics in dialysis is not similar to kinetics of numerous small solutes built up in uraemia (29), whereas Kt/V only employs the level of urea clearance for evaluation of dialysis adequacy (30).…”

American Society of Nephrology Quiz and Questionnaire 2015: ESRD/RRT

“…Dialysis adequacy, as determined by Kt/V urea, is measured differently for HD and PD and cannot be directly compared (choice B is incorrect). Furthermore, using urea clearance and values derived from urea kinetic models, such as Kt/V, as the sole gold standard for assessing dialysis adequacy continues to be vigorously debated (13,14). Although Kt/V can be easily obtained as a performance measure and relevant for policymakers, administrators, and insurers, more patient-relevant variables, such as a patient's overall wellbeing and how they are feeling-although more challenging to objectively measure-are arguably more clinically important indicators of optimal and not just adequate dialysis.…”