Evaluation of an in-line dialysate urea monitor including assessment of 'V'

Summary: Dialysate based assessment of the delivered dose of dialysis offers several advantages over blood sampling methods as it remains accurate in the face of fistula recirculation, urea rebound, variable blood flow and incorrect treatment time. Kr/V is calculated from the slope of the decline in urea concentration in the dialysate over the course of the treatment. By equilibrating dialysate with blood at the initiation of the treatment an estimate of pre-dialysis blood urea and the volume of distribution for urea (V) can be obtained. We performed Kt/V assessments on 20 in-centre haemodialysis patients using the Baxter Biostat 1000® dialysate urea monitor and compared the results with urea reduction ratios and Kt/V calculated by the formula of Basile. In addition, in 11 of these patients, V and pre-dialysis urea was derived and compared to total body water estimates using D₂O. the mean Kt/V by Biostat was 1.11 ± 0.23 and by formula was 1.23 ± 0.16 ( P < 0.005, Student's paired t -test). the lab pre-dialysis urea was 24.4 ± 6.2 mmol/L compared to the Biostat result of 23.9 ± 5.9 (when corrected for plasma water), with the mean difference of the techniques being −0.53 mmol/L. (95% CI −0.36–1.42). For V, the D₂O result was 36.7 ± 9.7 litres, compared to the Biostat result of 37.9 ± 9.6. the mean difference of the techniques by Bland and Altman analysis (or bias of the Biostat) was 1.2 L (95% CI −0.9–3.3) and the limits of agreement were −5.2–7.6L. Thus the Biostat provides easy access to dialysis adequacy data and gives a reasonable assessment of V, tending to overestimate this value.