Hemodialysis is prescribed for over 400,000 patients worldwide, often with little knowledge of the per treatment efficiency as related to the individual patients needs. A methodology and apparatus are proposed for the quantification of urea removal during treatment. A prototype urea sensor will be developed based on the marriage of immobilized enzyme and ion selective electrode technologies. The sensor design will allow continuous operation on-line in the dialysate stream. When adapted to a clinical setting (Phase II program) the concept as proposed will avoid the need for blood sampling.