Effect of treatment spacing and frequency on three measures of equivalent clearance, including standard Kt/V.

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 (G/TAC), mean predialysis UN (G/meanpre or 'standard' Kt/V) or peak UN (G/peak) 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 ml/min, session length = 210 min (105 min for 6/week), G = 7 mg/min, V = 35 l and weight gain = 10 l/week. RESULTS The 'standard' Kt/V (G/meanpre) was the same with the poorly spaced (Monday-Tuesday-Wednesday) and well-spaced (Monday-Wednesday-Friday) schedules. In contrast, the G/TAC- and G/peak-based clearances were higher with the well-spaced schedule (+20% and +37%, respectively). When total treatment time was held constant at 630 min/week, the gain of moving from three to six treatments per week was lower with G/TAC (+9%) than with G/meanpre (+29%) or with G/peak (+18%). When 6/week treatment time was doubled to 1260 min/week, the gains with G/TAC and G/pre (relative to 3/week with 630 min/week dialysis) were similar (about +94% to 97%), but were lower with G/peak (+55%). CONCLUSIONS All three equivalent clearances increased on moving from three to six sessions per week, with standard Kt/V having the greatest increase. Standard Kt/V 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.