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What volume to choose to assess online Kt/V?

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Abstract

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%.

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Funding

No funding agency granted the present study.

Author information

Authors and Affiliations

  1. Clinical Research Branch, Division of Nephrology, Miulli General Hospital, Acquaviva delle Fonti, Italy

    Francesco Gaetano Casino, Luigi Rossi & Carlo Basile

  2. Dialysis Centre SM2, Policoro, Italy

    Francesco Gaetano Casino

  3. Policlinico S.Orsola-Malpighi, Bologna, Italy

    Elena Mancini

  4. Ospedale Madonna delle Grazie, Matera, Italy

    Giovanni Santarsia & Salvatore Domenico Mostacci

  5. Ospedale Di Venere, Bari, Italy

    Filomena D’Elia & Maria Di Carlo

  6. Arcispedale S. Maria Nuova, Reggio Emilia, Italy

    Francesco Iannuzzella

  7. Ospedale Perrino, Brindisi, Italy

    Luigi Vernaglione

  8. Ospedale degli Infermi, Rimini, Italy

    Daniela Grimaldi

  9. Ospedale Nuovo, Imola, Italy

    Renato Rapanà

  10. Associazione Nefrologica Gabriella Sebastio, Via Battisti 192, 74121, Taranto, Martina Franca, Italy

    Carlo Basile

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  1. Francesco Gaetano Casino
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  2. Elena Mancini
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Correspondence to Carlo Basile.

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Research involving human participants and/or animals

Statement of human rights (1) The study has been conducted in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki. (2) Statement on the welfare of animals This article does not contain any studies with animals performed by any of the authors.

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Appendix

Appendix

SPEEDY is a user-friendly spreadsheet able to provide clinically acceptable results in HD prescription [27]. Specifically, V is calculated from the ratio K*T/spKt/V, as shown above for the calculation of V_KTV. However, K is estimated from KoA with the same equations of Solute-solver [18]. Furthermore, to take into account also Kru, if any, K*T becomes: (Kd + Kru)*T. spKt/V is calculated with a more recent equation, to take into account different interdialysis intervals preceding the session (PIDI) [29]. Finally, the calculated V (as well as V_KTV), which is a spVV, is transformed into a dpVV by means of the formula by Daugirdas and Smye [30]. To calculate V as V_SPEEDY in the present study, the original version of the software has been modified in order to use ID as direct input (simply by deleting the lines containing the equations that estimate Kd as a function of KoA). Similarly, a direct input of Kru is foreseen, by deleting the equations that calculate Kru [27].

For the sake of clarity, the equations used to calculate V_SPEEDY are:

  1. 1.

    spKt/V =  − LN(R-0.0174/PIDI * Td/60) + (4−3.5*R) * (BW0 − BWT)/BWT (where, R = BUNpost/BUNpre; PIDI = Preceding Interdialysis Interval, days, Td = session length, min

  2. 2.

    Single pool urea distribution volume (Vsp) = (Kd + Kru) * Td/spKt/V

  3. 3.

    Equilibrated Kt/V (eKt/V) = spKt/V * (Td/(Td + 30.7))

  4. 4.

    FDP1 = CT/EXP(LN(C0) − (eKt/V)/((spKt/V)/LN(C0/CT))) (FDP1 is an intermediate variable, CT = BUNpost, C0 = BUNpre)

  5. 5.

    Urea distribution volume ratio (Vratio) = LN(FDP1*C0/CT)/[FDP1*Ln(C0/CT)]

  6. 6.

    Double pool urea distribution volume (Vdp) = Vsp/V_ratio = V_SPEEDY.

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Casino, F.G., Mancini, E., Santarsia, G. et al. What volume to choose to assess online Kt/V?. J Nephrol 33, 137–146 (2020). https://doi.org/10.1007/s40620-019-00636-9

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