The lacking equation that estimates the protein catabolic rate in patients on once-weekly haemodialysis.

BACKGROUND: The normalized protein catabolic rate (PCRn) is one of the key indices derived from the urea kinetic model (UKM) in haemodialysis (HD) patients. Ideally, it should be assessed using the double pool UKM (KDOQI clinical practice guidelines, AIKD, 2015), as the web-based software Solute-Solver (SS) does (Daugirdas et al., AJKD, 2009). Simple formulae exist to compute PCRn for patients on thrice- or twice-weekly HD schedule, but not for patients on once-weekly HD schedule (1HD/wk). Aim of the present technical note was to introduce the lacking equation that estimates PCRn in the 1HD/wk regimen. METHODS: Data of a single HD session associated to monthly UKM studies were retrieved from the electronic database of our dialysis unit for 80 historical patients on 1HD/wk regimen. The UKM parameters, as calculated with SS, were used in a subgroup of 40 randomly selected patients (group 1) to build-up a multiple regression model of PCRn. The latter was used to predict PCRn (PCRnPred) values in the cohort of the remaining 40 patients (group 2). The Bland-Altman plot was used to analyse the agreement between PCRnPred and the paired "observed" (PCRnObs) values, as measured with SS. RESULTS: The following equation was established by means of the multiple regression analysis: PCRn = - 0.46 + 0.01 × C0 + 0.09 × eKt/V + 3.94 × Kru/V, where C0 is pre-dialysis blood urea nitrogen concentration, eKt/V is the equilibrated Kt/V, Kru is the residual renal urea clearance and V is the post-dialysis urea distribution volume. The PCRnPred values were 0.99 ± 0.24 g/kg/day; the PCRnObs values were 0.96 ± 0.23 g/kg/day (mean difference 0.03 ± 0.05 g/kg/day). Their difference at the Bland-Altman analysis ranged from - 0.08 to + 0.13 g/kg/day. Finally, a nomogram was drawn: it can be used to estimate not only PCRn from Kru/V and C0, but also C0 as a function of Kru/V and PCRn. CONCLUSIONS: The equation here introduced allows a simple and accurate estimate of PCRn in patients on once-weekly HD regimen. The availability of the nomogram relating C0 to PCRn and Kru/V could be a further step to make safer and safer the once-weekly HD regimen. The following equation was established by means of the multiple regression analysis [Formula: see text] where PCRn is the normalized protein catabolic rate (PCRn), C0 is pre-dialysis blood urea nitrogen concentration (BUN), eKt/V is the equilibrated Kt/V, Kru is the residual renal urea clearance and V is the post-dialysis urea distribution volume. A nomogram relating pre-dialysis BUN to PCRn and Kru/V could be drawn: it can be used to estimate not only PCRn from Kru/V and pre-dialysis BUN, but also pre-dialysis BUN as a function of Kru/V and PCRn.

What volume to choose to assess online Kt/V?

INTRODUCTION: Urea distribution volume (V) can be assessed in different ways, among them the anthropometric Watson Volume (VW). However, many studies have shown that VW does not coincide with V and that the latter can be more accurately estimated with other methods. The present multicentre study was designed to answer the question: what V to choose to assess online Kt/V? MATERIALS AND METHODS: Pre- and postdialysis blood urea nitrogen concentrations and the usual input data set for urea kinetic modelling were obtained for a single dialysis session in 201 Caucasian patients treated in 9 Italian dialysis units. Only dialysis machines measuring ionic dialysance (ID) were utilized. ID reflects very accurately the mean effective dialyser urea clearance (Kd). Six different V values were obtained: the first one was VW; the second one was computed from the equation established by the HEMO Study to predict the single pool-adjusted modelled V from VW (VH) (Daugirdas JT et al. KI 64: 1108, 2003); the others were estimated kinetically as: 1. V_ID, in which ID is direct input in the in the double pool variable volume (dpVV) calculation by means of the Solute-solver software; 2. V_Kd, in which the estimated Kd is direct input in the dpVV calculation by means of the Solute-solver software; 3. V_KTV, in which V is calculated by means of the second generation Daugirdas equation; 4. V_SPEEDY, in which ID is direct input in the dpVV calculation by means of the SPEEDY software able to provide results quite similar to those provided by Solute-solver. RESULTS: Mean± SD of the main data are reported: measured ID was 190.6 ± 29.6 mL/min, estimated Kd was 211.6 ± 29.0 mL/min. The relationship between paired data was poor (R2 = 0.34) and their difference at the Bland-Altman plot was large (21 ± 27 mL/min). VW was 35.3 ± 6.3 L, VH 29.5 ± 5.5, V_ID 28.99 ± 7.6 L, V_SPEEDY 29.4 ± 7.6 L, V_KTV 29.7 ± 7.0 L. The mean ratio VW/V_ID was 1.22, (i.e. VW overestimated V_ID by about 22%). The mean ratio VH/V_ID was 1.02 (i.e. VH overestimated V_ID by only 2%). The relationship between paired data of V_ID and VW was poor (R2 = 0.48) and their mean difference at the Bland-Altman plot was very large (- 6.39 ± 5.59 L). The relationship between paired data of V_ID and VH was poor (R2 = 47) and their mean difference was small but with a large SD (- 0.59 ± 5.53 L). The relationship between paired data of V_ID and V_SPEEDY was excellent (R2 = 0.993) and their mean difference at the Bland-Altman plot was very small (- 0.54 ± 0.64 L). The relationship between paired data of V_ID and V_KTV was excellent (R2 = 0.985) and their mean difference at the Bland-Altman plot was small (- 0.85 ± 1.06 L). CONCLUSIONS: V_ID can be considered the reference method to estimate the modelled V and then the first choice to assess Kt/V. V_SPEEDY is a valuable alternative to V_ID. V_KTV can be utilized in the daily practice, taking also into account its simple way of calculation. VW is not advisable because it leads to underestimation of Kt/V by about 20%.