Estimating Time-averaged Serum Urea Nitrogen Concentration during Various Urine Collection Periods: A Prediction Equation for Thrice Weekly and Biweekly Dialysis Schedules

Residual kidney urea clearance in dialysis patients typically is calculated as the per minute excretion of urea nitrogen, obtained during the 24-48 hour collection period that usually ends just prior to a dialysis session, divided by the time-averaged serum water urea nitrogen concentration during the collection period. This concentration is difficult to estimate unless a formal kinetic modeling program is being used. We used a urea kinetic modeling program to derive an equation to estimate the time-averaged serum water concentration during urine collection periods of various lengths collected during various interdialytic intervals, for 3/week or 2/week dialysis schedules. The equation to predict time-averaged serum water urea nitrogen concentration during the collection period was a function of the ratio of the duration of the predialysis collection period to the duration of the interdialytic interval during which the collection was accomplished, as well as the urea reduction ratio and predialysis serum urea nitrogen concentration of the dialysis session immediately following the end of the collection period.