[Dialysis adequacy: Kt/V on its way out?].

The Kt/V index started being used in the 1980s to determine the adequacy of dialysis treatment. Based on a pharmacokinetic model and on urea as a marker of uremia, its use in clinical practice has allowed the development of new dialyzers that can provide high diffusive clearance but, above all, has established the knowledge that dialysis treatment should be tailored to the characteristics of the patient. After an initial period of uncertainty in which it was discussed which formula or model should be employed, there came a phase of excitement in which it was believed that the Kt/V index was synonymous with adequate dialysis. In the following years, several studies highlighted the limitations of Kt/V; in particular, the rescaling of Kt for the volume of distribution is a confounding factor because in patients on dialysis, sex and body mass index per se affect the mortality and morbidity. Doubts were also raised about urea as a marker of uremia. Whereas the measurement of Kt/V urea should not be abandoned, it should be viewed in the broader context of the definition of ''adequate dialysis'' as an index of the removal of small molecules, possibly together with Kt/V B2M as an index of the removal of middle molecules.