Evaluation of urea reduction ratio estimated from the integrated value of urea concentrations in spent dialysate

We propose a new apparatus and method to estimate accurate urea reduction ratio without influence of any rebound. For this purpose, we have developed an improved version of a chemiluminescence-based urea sensor capable of measuring urea concentration in spent dialysate (CD ) at 2-min intervals. The correlation coefficient between the readouts of the sensor and the conventional enzyme-UV method was 0.97 and the sensor was not affected by 9 mmol/L uric acid, creatinine, or ammonia. Using the urea sensor, CD was measured as a function of dialysis time t during dialysis sessions for various blood flow rates. The urea reduction ratio based on the mass of the urea reduced (m URR) was estimated from the integrated value of CD (t). When in vitro urea concentrations are measured during a dialysis session in model blood (urea solution) at a constant volume (V) in a vessel, the plots of m URR were congruous with theoretical curves of conventional URR calculated based on the one-pool model, and thus the accuracy of m URR was confirmed. On the other hand, in in vivo measurements of CD (t) during dialysis treatment for two patients, the plots of the m URR were not congruous with the theoretical curves of URR. Such a difference between URR and m URR was explained by the deviation of actual dialysis from the one-pool model, and it was concluded that m URR might be accurate under any dialysis condition.