Quantifying the discontinuity of haemodialysis dose with time-averaged concentration (TAC) and time-averaged deviation (TAD)

It is with great interest that we have read the analysis of Daugirdas et al. on the effects of spacing of discontinuous haemodialysis on weekly dose of dialysis [1]. The authors showed that (contrary to other measures) the change in maximum concentration (‘peak’) associated with intermittent dialysis and attained at the end of the long dialysis interval best captured the increase in the weekly dose, especially when changing from illto well-spaced treatments. Long before the introduction of equivalent renal clearance [2] and so-called standard Kt/V [3], Lopot et al. compared different modes of dialysis by computing the timeaveraged deviation (TAD) and plotted this measure against the time-averaged concentration (TAC) of urea [4,5]. For c (t), representing the time-dependent urea nitrogen concentration (in millimole per litre), and T, representing the duration of 1 week (10 080 min), TAC and TAD are given as