Urea kinetic analysis of automated peritoneal dialysis allows calculation of a CAPD-equivalent Kt/V(urea)

Based on evidence of increased mortality with decreasing urea clearance, the Dialysis Outcomes Quality Initiative (DOQI) recommended a weekly Kt/Vurea of 2.0 or higher for patients receiving continuous ambulatory peritoneal dialysis (CAPD). DOQI recommendations for automated peritoneal dialysis (APD) are based on efforts to determine the clearance providing urea mass removal equivalent to CAPD. We have adapted a variable volume direct quantitation urea kinetic model (UKM) in an effort to assess DOQI APD guidelines.

The daily urea mass removed with a weekly Kt/Vurea of 2.0 was calculated using standardized CAPD patient profiles. Using this value and defining the pre-APD plasma urea nitrogen (PUN) as C0 and equal to the CAPD steady-state PUN, the UKM reiteratively calculated the urea clearance from an APD treatment that provided a urea mass removal equivalent to CAPD. A total weekly Kt/Vurea was calculated for various levels of continuous urea clearance (defined as Kprt/Vurea) and plotted against Kprt/Vurea (weekly). The impact of dialytic time (t), drain volume of the daytime dwell (delta), and ultrafiltration volume (phi) were assessed, and all profiles were performed with C0 equal to the corresponding blood urea nitrogen of 60, 70, and 80 mg/dL.

The relationship between requisite weekly Kt/Vurea and Kprt/Vurea (weekly) was linear. Weekly Kt/Vurea declined with increasing Kprt/Vurea, t, delta, and phi. The effect of phi on the weekly Kt/Vurea was independent of Kprt/Vurea, and the magnitude of the effect of t and delta on the weekly Kt/Vurea decreased with increasing continuous clearance. Weekly Kt/Vurea values were independent of V and C0. The latter observation allowed extrapolation of CAPD clearance and urea generation relationships to APD: CAPD-equivalent weekly Kt/Vurea = [700 x (UD + Ur)]/(C0 x V), where UD and Ur are the daily urea mass (mg) in dialysate and urine, respectively.

The APD urea clearance, which provides urea mass removal equivalent to CAPD, varies as a function of a combination of patient and treatment variables. However, a CAPD-equivalent weekly Kt/Vurea can be calculated by collecting appropriate dialysis and urine samples and estimating patient V. The results can be evaluated in the context of evidence-based CAPD guidelines, increasing the precision of adjustment and monitoring of the APD prescription.