Effect of treatment spacing and frequency on three measures of equivalent clearance, including standard KtV

Abstract

Background. We examined the sensitivity of three different equivalent clearances to the spacing of haemodialysis treatments as well as to frequency. One would expect that a well-spaced schedule would be beneficial, and an optimal clearance measure should reflect this.Methods. Using a variable volume two-pool urea kinetic model, we derived clearances based on G urea nitrogen (UN) generation rate divided by time-averaged UN (GTAC), mean predialysis UN (Gmeanpre or 'standard' KtV) or peak UN (Gpeak) when identical dialysis treatments were given on a poorly spaced (Monday-Tuesday-Wednesday) versus a well-spaced (Monday-Wednesday-Friday) schedule. We also calculated the 'gain' in each clearance when well-spaced treatments were given six versus three times a week. Modelling parameters were diffusive dialyser clearance = 283 mlmin, session length = 210 min (105 min for 6week), G = 7 mgmin, V = 35 l and weight gain = 10 lweek.Results. The 'standard' KtV (Gmeanpre) was the same with the poorly spaced (Monday-Tuesday-Wednesday) and well-spaced (Monday-Wednesday- Friday) schedules. In contrast, the GTAC-and Gpeak-based clearances were higher with the well-spaced schedule (+20 and +37, respectively).When total treatment time was held constant at 630 minweek, the gain of moving from three to six treatments per week was lower with GTAC (+9) than with Gmeanpre (+29) or with Gpeak (+18). When 6week treatment time was doubled to 1260 minweek, the gains with GTAC and Gpre (relative to 3week with 630 minweek dialysis) were similar (about +94 to 97), but were lower with Gpeak (+55).Conclusions. All three equivalent clearances increased on moving from three to six sessions per week, with standard KtV having the greatest increase. Standard KtV is not at all sensitive to spacing. Alternative clearances based on the TAC or peak concentration have the advantage of taking both spacing and frequency into account.