Prediction equation for calculating residual kidney urea clearance using urine collections for different hemodialysis treatment frequencies and interdialytic intervals

The purpose of the study was to explore the precision of an equation designed to estimate residual kidney urea clearance (K-RU) from interdialytic urine collection data and pre-hemodialysis (HD) serum urea nitrogen (SUN) in different hemodialysis treatment schedules. The generalizability of the proposed equation was tested in 32 731 HD treatments where urine was collected prior to a dialysis session, mostly for 24 h but sometimes longer, in patients being dialyzed 1-4 times/week. The residual kidney urea clearance estimating equation predicted a K-RU that matched the one computed by formal modeling within 5% in > 98% of sessions analyzed. The errors in estimated versus modeled K-RU for interdialytic intervals (IDIs) of 2, 3, 4 and 7 days, were 1.6 +/- 1.5%, -0.4 +/- 1.6%, 0.9 +/- 1.6%, and 1.5 +/- 1.2%, respectively. Percent errors were similar for schedules of 1-4/week with the exception of urine collection during the 2-day interval of a 2:5-day twice-weekly schedule; here error averaged 5.0 +/- 1.2%. Use of the average of the SUN values at the start and end of the collection period overestimated modeled K-RU by 11.3 +/- 4.5%, whereas an equation suggested by others underestimated modeled K-RU by -9.9 +/- 3.4%. The equation tested predicts values for K-RU that are similar to those obtained from formal urea kinetic modeling, with percent errors that only rarely exceed 5%. It gives relatively precise results for a wide range of HD treatment schedules, IDIs and urine collection periods.