On-line urea kinetics in haemodiafiltration.

BACKGROUND

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. Methods. 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.

RESULTS

Urea nitrogen concentrations were highly correlated to the difference (Delta) 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=-637+32.01x, R2=0.98, P<0.00001). Delta conductivity was highly reproducible (% variation: ).8-5.3%) and stable (maximal % variation at 150 mg/dl after 100 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).

CONCLUSIONS

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