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NDT Advance Access published online on August 12, 2008
Nephrology Dialysis Transplantation, doi:10.1093/ndt/gfn457
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© The Author [2008]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org


A comparison of methods for determining urea distribution volume for routine use in on-line monitoring of haemodialysis adequacy

Elizabeth J. Lindley1, Paul W. Chamney2, Andreas Wuepper2, Helen Ingles1, James E. Tattersall1 and Eric J. Will1

1 Department of Renal Medicine, St James's University Hospital, Leeds, UK 2 Fresenius Medical Care, Research & Development, Bad Homburg, Germany

Correspondence and offprint requests to: Elizabeth J. Lindley, Department of Renal Medicine, St James's University Hospital, Beckett Street, Leeds LS9 7TF, UK. Tel: +44-113-206-4199; Fax: +44-113-206-6064; E-mail: Elizabeth.Lindley{at}leedsth.nhs.uk



  Abstract

Background. The availability of haemodialysis machines equipped with on-line clearance monitoring (OCM) allows frequent assessment of dialysis efficiency and adequacy without the need for blood samples. Accurate estimation of the urea distribution volume ‘V’ is required for Kt/V calculated from OCM to be consistent with conventional blood sample-based methods.

Methods. Ten stable HD patients were monitored monthly for 6 months. Time-averaged OCM clearance (KOCM) and pre- and post-dialysis blood samples were collected at each monitored session. The second generation Daugirdas formula was used to calculate the single-pool variable volume Kt/V, (Kt/V)D. Values of V to allow comparison between OCM and blood-based Kt/V were determined from Watson's formula (VWatson), bioimpedance spectroscopy (VBIS), classical urea kinetic modelling (VUKM_C) and a simple computation of V (VUKM_S) from the blood-based Kt/V and KOCMt.

Results. Comparison of KOCMt/V with (Kt/V)D shows that using VWatson leads to significant systematic underestimation of dialysis dose. KOCMt/VBIS agrees with (Kt/V)D to within ± 10%. KOCMt/VUKM_S is, by definition, identical to (Kt/V)D when initially calculated. However, if a historical value of V is used, agreement between KOCMt/V and (Kt/V)D over 6 months varies by 5% for VBIS and 10% for VUKM_S.

Conclusions. When investigating the effect of different treatment strategies on dialysis efficiency, any estimate of V can be used provided it is constant, as K is the relevant parameter. When frequent supervision of actual dialysis dose is required, the greatest consistency between KOCMt/V and the reference, Kt/VD, over time is achieved with VBIS.

Keywords: haemodialysis adequacy; on-line clearance monitoring; urea distribution volume; urea kinetic modelling

Received for publication: 26. 5.07
Accepted in revised form: 16. 7.08


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