A disposable urea sensor for continuous monitoring of hemodialysis efficiency

A new sensor for the on-line measurement of urea in the dialysate output is described. The sensor is based on a differential measurement of conductivity changes induced by the urease-catalyzed hydrolysis of urea. The use of screen printing for the batch-fabrication of the basic transducers results in cheap disposable devices. In addition, the sensor has been designed to fit into a standard male luer-adapter, and can be plugged directly into the dialysate line. The in vitro response is linear to urea concentrations exceeding 6620mM. A resolution of 20020 microM has been achieved with a baseline stability of 50 microM/hr. Interferences caused by fluctuations in the ionic strength and the consequent conductivity changes are effectively suppressed by the differential sensor pair. The efficiency of this suppression is expressed in a common mode rejection ratio of typically 40 to 50. Preliminary ex vivo results show the feasibility of the concept. The sensor principle is not restricted to urea but can be extended to other molecules of interest for hemodialysis monitoring, such as creatinine and L- and D-amino acids.