Measures of Equivalent Hemodialysis Urea Clearance and Their Proposed Utility for Monitoring Adequacy

The details of four measures of "equivalent" clearances based on urea that have been proposed to monitor hemodialysis adequacy are reviewed. These measures fall into two groups: one based on urea nitrogen generation rate divided by the time-averaged serum urea nitrogen concentration, and the other based on urea nitrogen generation rate divided by the average predialysis serum urea nitrogen concentration. After the original variants were proposed, a potential problem regarding relative weights of dialysis and residual kidney clearances was identified, and revised equivalent urea clearances were proposed that give a higher weight to residual kidney clearance relative to dialysis clearance. These measures suggest that the target dialysis dose should be set to achieve dialysis solute concentrations typically found in patients at the point of needing dialysis, e.g., with a glomerular filtration rate of around 10 mL/min, corresponding to a urea clearance of approximately 6 mL/min and a creatinine clearance of 14. Despite the theoretical attractiveness of equivalent urea clearances to monitor hemodialysis adequacy, their utility has not been conclusively demonstrated in observational studies of incremental or more frequent hemodialysis.