The efficacy of solute removal by renal replacement therapy can be assessed by the commonly used index of KT/V (the fraction of the volume cleared from a solute). Fractional solute removal (FSR, the fraction of the total amount of the solute that was removed) is an alternative index that may be more appropriate than KT/V for comparison of the efficacy of different treatment modalities. To elucidate the relationship between these two indexes, we propose to discriminate between two notions of clearance: (1) instantaneous clearance K = (solute removal rate)/CB, where CB is solute concentration in blood, and (2) treatment clearance KT = (average rate of solute removal per treatment)/CB0, where CB0 is CB at the beginning of the treatment. K is the clearance of the purification device (glomeruli, hemodialyzer or hemofilter) and the diffusive mass transport parameter (KBD, MTAC) for continuous ambulatory peritoneal dialysis (CAPD). For all modalities of renal replacement therapy: FSR = KTT/V, and KT generally decreases with the treatment time. For purification of a single compartment with a constant volume, V, using an open loop system (i.e. with no recirculation or dwelling of dialysis fluid, as in hemodialysis (HD), hemofiltration (HF) or in the native kidney), FSR is a function of only one lumped, nondimensional parameter, KT/VB, where VB is the distribution volume of the solute within the body. In contrast, if closed loop systems are applied, as for example in HD with recirculation of dialysis fluid (RD) or in peritoneal dialysis, FSR depends on two lumped, nondimensional parameters: KT/VB and KT/VD, where VD is the volume of dialysis fluid. It is necessary to discriminate between K and KT for analysis of dialysis dose. For HD and HF, FSR is a function of KT/V, whereas KT/V alone does not allow calculation of FSR for CAPD and RD. The current practice of using KTT/V for CAPD but KT/V for HD and HF leads to confusion because of the inconsistency in the interpretation of the quantitative prescription of dialysis dose. The application of FSR, instead of KT/V, for all treatment modalities may solve this dilemma. Furthermore, KTT/V (currently used only for CAPD) is equal to FSR for all treatment modalities. Both FSR and KT may be generalized to describe the total solute removal per treatment cycle composed from a few treatment sessions. A few different definitions of the adequacy parameters for the treatment cycle are formulated and discussed.

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