KT/V: the denominator dilemma

The efficacy of solute removal by renal replacement therapy can be assessed by the commonly used index of KT/V. However, KT/V has a different meaning in hemodialysis (HD) and peritoneal dialysis (PD), and KT/V in HD and PD can therefore not be compared. For example, in HD one uses an instantaneous clearance K = (solute removal rate)/CB, where CB is solute concentration in blood. Thus, K is the clearance of the purification device (hemodialyzer) and in continuous ambulatory peritoneal dialysis (CAPD) it is the diffusive mass transport parameter (KBD, MTAC) but this is not used in clinical practice. Instead, in CAPD one uses a treatment clearance KT = (average rate of solute removal per treatment)/CBO, where CBO is CB at the beginning of the treatment. Whereas K is constant, KT decreases during the CAPD dwell. The current practice of using KTT/V for CAPD but KT/V for HD leads to confusion. Furthermore, T is different in CAPD (about 150 hours per week) and HD (about 12 hours per week). Finally, V, the distribution volume of the solute (usually urea), is calculated in different ways in HD and PD. V is strongly related to nutritional status and as such it is a strong predictor of survival. KT in HD, and KTT in CAPD, represents the dialysis dose and as such also a predictor of survival. This may at least in part explain why the recent Ademex study in PD patients and the Hemo study in HD patients could not demonstrate any strong impact of different levels of KT/V (KTT/V) on clinical outcome. In this review, we discuss the shortcomings of the "KT/V" concept, in particular its limited value for comparisons between dialysis efficiency in PD and HD, as well as the dilemma of using V as the denominator considering that V in itself may represent a strong predictor of outcome.