Urea Reduction Ratio - Limitations of URR Vs. Kt/V

Limitations of URR Vs. Kt/V

The URR is designed to measure the amount of dialysis given when the dialysis clearance of urea greatly exceeds the urea generation rate. In continuous hemodialysis or in peritoneal dialysis, for example, a considerable amount of dialysis is delivered, but the urea level remains roughly constant after the initial treatment of uremia, so the URR is essentially zero. In long slow overnight dialysis, if simplified equations are used, the URR also underestimates the amount of dialysis due to urea generation during the long dialytic session. For this reason, the kinetically modeled Kt/V is always recommended as the best measure of dialysis adequacy. The Kt/V, even that derived by formal modeling, is primarily based on the URR, and so it contains little additional information in terms of the amount of dialysis that was delivered. Since the URR and Kt/V are so closely related, their predictive power in terms of patient outcome is similar. However, use of Kt/V and urea modeling in general allows for comparing expected with predicted dose of dialysis, which can be used to analyze dialysis treatments and dialyzer clearances and in troubleshooting and quality control activities. Also, Kt/V permits calculation of the urea generation rate, which can give clues about a patient's protein intake.