Do we need another Kt/V?

A new method of estimating Kt/V has been proposed. This was a required update to accommodate frequencies of dialysis other than thrice weekly [1].The original approximate method for calculating Kt/V from preand post-dialysis urea concentrations was developed using linear regression on a data set of measurements taken from patients undergoing thrice-weekly dialysis. The method assumes that the interval between the blood samples and preceding dialysis was 2 days. Unfortunately, when dialysis is delivered other than over a standard thrice-weekly schedule, the existing approximate method for Kt/V becomes inaccurate. A different correction factor for the urea generation rate is required, taking the frequency and duration of the preceding interdialytic interval into account. There are now at least 11 different ways of quantifying small-solute clearance in dialysis (Table 1), 7 of which are different kinds of Kt/V. In addition, there are at least five different approximation methods for calculating Kt/V (Table 2). Kt/V was initially used to quantify the dose of a single haemodialysis (HD) session in terms of urea clearance (K) and time (t) [2]. Kt/V is the exponential term describing the changing urea concentration during the dialysis session. Kt/V was proposed in the context of urea kinetic modeling (UKM), where the change in concentration of urea (C) could be predicted from K, generation rate (G) and V. Similarly, G and Kt/V could be calculated from the change in C. V or K can also be calculated by UKM if the other is known. UKM is useful for trouble shooting and quality control of intermittent dialysis as differences between expected and delivered Kt/V expose problems with the dialysis process. Another type of Kt/V, the equilibrated Kt/V (eKt/V) accounts for the transfer of urea within the patient and the postdialysis rebound. The eKt/V is always lower than Kt/V, and the difference is greater with shorter dialysis sessions, which have a greater post-dialysis rebound. Later, Kt/V was applied to peritoneal dialysis (PD) [3]. In this case, t is set to an arbitrary interval (usually 1 week). Since C is relatively stable, UKM is not possible and V is estimated using anthropometric methods (e.g. Watson) or measured using bioimpedance. V calculated in dialysis patients using bioimpedance is up to 30% lower than when predicted using Watson. V calculated using bioimpedance agrees with V calculated by UKM [4]. In a typical HD patient, blood urea concentrations during the entire week are more influenced by the urea generation rate (G), duration of the periods between dialysis and any renal function than they are by Kt/V. Therefore, Kt/V is not useful in comparing different types of dialyses and schedules. Schedules with shorter intervals between dialysis (e.g. long nocturnal, daily) result in a lower weekly peak and time average urea concentrations (TACs) than standard thrice-weekly dialysis despite similar Kt/V multiplied by the number of sessions per week. With thrice-weekly dialysis, in an anuric patient, even an infinite Kt/V would result in lower TAC than 12 mL/min of continuous urea clearance (e.g. by renal function). For a more realistic comparison of clearance delivered with varying schedules and taking renal function into account, the standard Kt/V (stdKt/V) has been proposed [5]. This quantifies dialysis as G divided by the average peak (pre-dialysis) concentration calculated by UKM. In an individual patient, treatments resulting in the same stdKt/V but with different schedules or even continuous treatments would have the same average peak urea concentrations. However, TAC and the highest peak concentration would still be higher in more intermittent treatments [6]. The unphysiological variations in concentration due to intermittent dialysis are also not taken into account in stdKt/V, but could be quantified as the time average deviation (TAD) [7]. Alternative continuous dose measures are calculated from G in the same way as stdKt/V, but use different, possibly more clinically relevant, measures of concentration than the average peak. The solute removal index (SRI) [8] uses the highest peak urea concentration. The equivalent renal urea clearance (EKR)