The authors have previously demonstrated the feasibility and accuracy of urea kinetic modeling (UKM) based on monitoring urea concentration in the spent dialysate stream (SDS) throughout the hemodialysis (HD) session. They describe here a prototype urea sensor for this purpose and initial experience with HD patients. The sensor is based on ammonium ion and reference electrodes housed in a cell through which the entire SDS passes. The two electrode tips are bathed in urease solution on one side of a dialysis membrane; the SDS flows along the adjacent side. Urea diffusing across the membrane from the SDS is converted by the urease into ammonium ion, which is measured by the electrode pair. For evaluation, the prototype flowthrough urea sensor was installed in the SDS of a Cobe Centry 3 HD machine for 36 HD sessions. Independent measurement demonstrated a linear relationship between mv output of the sensor and logarithm of SDS urea concentration. The use of SDS urea concentration time profiles obtained with this sensor to obtain accurate values of patient protein catabolic rate (PCR) and KT/V is illustrated. Incorporation of urea sensors such as this prototype into HD machines, will permit complete automation of UKM in the near future.
