The residual renal urea clearance (KrU) plays an important role in determining the patient's kinetically modeled dialysis prescription and in calculating the protein catabolic rate to assess dietary protein intake. Measurement of renal function, which requires an interdialytic urine collection and blood samples taken during two different dialyses, may be prone to collection and blood sampling omissions. This study compared the accuracy of residual KrU calculations using two different methods to determine the mean blood urea nitrogen (BUN) level during the urine collection interval. A comparison of residual renal creatinine clearance (KrCr) calculations was simultaneously evaluated. Interdialytic urine collections including measurements of urine urea nitrogen, creatinine, and total urine volume, were obtained from 30 patients during a 10-month period. The collection time was shortened from an entire interdialytic interval to 24 to 36 hours. Calculation of the mean blood levels using the first method required measurements of BUN and serum creatinine (Cr) at the end of the first dialysis and at the start of the second dialysis. The second method measured pre- and post-BUN and serum Cr during the second dialysis. A renal function program for the 41 CX programmable calculator (Hewlett Packard, Palo Alto, CA) was used to calculate 110 KrU and 88 KrCr measurements. The mean residual renal clearance values were identical using both methods (KrU = 2.8 ± 1.6 mL/min; KrCr = 5.5 ± 3.1 mL/min; P = NS). These results show that the residual KrU and KrCr can be accurately calculated from the measurement of a single dialysis pre- and post-BUN and serum Cr in conjunction with a urine collection. The residual renal urea clearance (KrU) plays an important role in determining the patient's kinetically modeled dialysis prescription and in calculating the protein catabolic rate to assess dietary protein intake. Measurement of renal function, which requires an interdialytic urine collection and blood samples taken during two different dialyses, may be prone to collection and blood sampling omissions. This study compared the accuracy of residual KrU calculations using two different methods to determine the mean blood urea nitrogen (BUN) level during the urine collection interval. A comparison of residual renal creatinine clearance (KrCr) calculations was simultaneously evaluated. Interdialytic urine collections including measurements of urine urea nitrogen, creatinine, and total urine volume, were obtained from 30 patients during a 10-month period. The collection time was shortened from an entire interdialytic interval to 24 to 36 hours. Calculation of the mean blood levels using the first method required measurements of BUN and serum creatinine (Cr) at the end of the first dialysis and at the start of the second dialysis. The second method measured pre- and post-BUN and serum Cr during the second dialysis. A renal function program for the 41 CX programmable calculator (Hewlett Packard, Palo Alto, CA) was used to calculate 110 KrU and 88 KrCr measurements. The mean residual renal clearance values were identical using both methods (KrU = 2.8 ± 1.6 mL/min; KrCr = 5.5 ± 3.1 mL/min; P = NS). These results show that the residual KrU and KrCr can be accurately calculated from the measurement of a single dialysis pre- and post-BUN and serum Cr in conjunction with a urine collection.