Journal Article

On-line urea kinetics in haemodiafiltration

A. Santoro, C. Tetta, S. Mandolfo, S. Arrigo, M. Berti, G. Colasanti, G. D'Amico, E. Imbasciati, C. Mazzocchi, G. Pacini, M. Spongano, K. Thomaseth, M. L. Wratten and P. Zucchelli

in Nephrology Dialysis Transplantation

Volume 11, issue 6, pages 1084-1092
Published in print June 1996 | ISSN: 0931-0509
Published online June 1996 | e-ISSN: 1460-2385 | DOI:
On-line urea kinetics in haemodiafiltration

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Calculation of Kt/V and assessment of nutrition have so far been dependent upon off-line urea measurements of blood or dialysate samples. Here we describe a biosensor for on-line urea measurement during haemodiafiltration.


The biosensor consisted of a cartridge containing covalently linked urease placed between two conductivity cells. The biosensor was placed on the outlet line of a haemofilter in series with a dialyser in order to obtain an aliquot of plasma ultrafiltrate for on-line measurement of urea.


Urea nitrogen concentrations were highly correlated to the difference (Δ) in conductivity measured by the two conductivity cells both in aqueous solutions (in-vitro studies, y=−6.676+32.12x, R2=0.998, P<0.0001) and in ultrafiltrates (ex-vivo studies, y=−6.7+32.01x, R2=0.98, P<0.00001). Δ conductivity was highly reproducible (% variation: 0.8–5.3%) and stable (maximal % variation at 150 mg/dl after 180 min: 0.9±0.3 vs initial values). The intradialytic plasma water urea profile was obtained in 10 haemodialysis patients. To study recirculation, the plasma water urea profile was analysed before and 3 min after stopping the dialysate flow. The pre- and post-stopped flow ratio (1.21±0.1, mean±1 SD) was superimposable to conventional blood sampling data (opposite arm venous/arterial: 1.22±0.11) and allowed correction for recirculation. A novel approach to urea kinetic modelling was described and used to reliably project end-dialysis and post-dialysis rebound urea concentration as early as 90 min. Projected (29.2±10.4 g) or measured (29.8±10.5 g) net urea removal was highly correlated with the amount of urea collected in the total spent dialysate (29.7±10.6 g) (R2=0.99, R2=0.97 respectively).


These results indicate that on-line, real-time analysis of urea kinetics may provide information on delivery of adequate dialysis in high-efficiency techniques.

Keywords: biosensor; haemodialysis; mathematical modelling; on-line monitoring; recirculation; urea kinetics

Journal Article.  0 words. 

Subjects: Nephrology

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