Dialysate to plasma solute concentration (D/P) versus peritoneal transport parameters in CAPD

The relationship between dialysate to plasma solute equilibration ratio (D/P) and diffusive (diffusive mass transport coefficients, KBD) as well as convective (sieving coefficient, S, and net ultrafiltration) transport characteristics were studied in clinically stable CAPD patients and in patients with loss of ultrafiltration capacity (UFC). Forty-one 6-h single-dwell studies with standard glucose-based dialysis fluids containing 1.36% (n = 9), 2.27% (n = 9), and 3.86% (n = 23) anhydrous glucose were carried out in 33 clinically stable CAPD patients. Eleven patients with loss of UFC were studied with 3.86% glucose solution. Intraperitoneal dialysate volumes were calculated from the dilution of the tracer (131I-albumin) with a correction applied for its elimination from the peritoneal cavity. KBD and S were estimated using the Pyle-Popovich-Moncrief model with aqueous plasma concentrations. A theoretical D/P curve was derived with and without taking convective transport and peritoneal reabsorption into account. The three different glucose solutions yielded D/P curves which were similar for urea and potassium. For creatinine a slower equilibration and for glucose and sodium a faster decrease in dialysate concentration were observed with more hypertonic solutions. In general, there was a strong correlation which was best at 240 min between D/P (for glucose dialysate/initial dialysate concentration, D/Do) and KBD for all solutes (except sodium), whereas the correlation between D/P and convective transport parameters was much weaker. KBD for creatinine (with 3.86% glucose solution) could be estimated (r = 0.98) from aqueous D/P using the experimental formula: creatinine KBD = -1.8 - ln (1 - D240/P)/0.1. Patients with loss of UFC due to increased diffusive transport (n = 8) could be discriminated from the clinically stable patients using KBD and D/P (or D/D0) for creatinine and glucose or D/P for sodium. However, patients with loss of UFC associated with increased peritoneal reabsorption (n = 2) could not be identified using these parameters. Theoretically derived D/P curves were in excellent agreement with measured D/P for 1.36% glucose solution and simulations were satisfactory also for the 2.27% and 3.86% solutions provided that the effect of convective transport was taken into account. The standardized peritoneal equilibration tests (PET) as proposed by Twardowski et al. [17] seems to be appropriately designed as regards duration of the dwell and the choice of glucose and creatinine as investigated solutes. Thus, PET can be recommended as a sensitive routine investigation for the monitoring of normal/abnormal peritoneal transport behaviour in peritoneal dialysis patients.