It is now well appreciated that peritoneal dialysis patients have different peritoneal membrane transport characteristics. These differences are best classified and determined by use of the peritoneal equilibration test (PET) . (See "Peritoneal equilibration test".) If the standard PET is done and all recommended measurements obtained, this test has helped characterize the relationship between dwell time, solute transport, glucose absorption, drain volume, and net solute removal. Those patients who have the highest rates of diffusive transport are classified as rapid transporters. According to PET testing in different populations, approximately 15 percent of patients will be rapid transporters at the start of peritoneal dialysis.
As a result of the high rates of diffusive transport, rapid transporters transport small solutes (such as urea, creatinine, and glucose) quickly, leading to equilibration between the dialysate small-solute concentration and that of the blood relatively early in a dwell (figure 1). These patients also rapidly absorb dialysate glucose, leading to early dissolution of the crystalloid osmotic gradient between dialysate and blood that is required to sustain ultrafiltration. Once the osmotic gradient is dissipated, the stimulus for ultrafiltration is gone, and ultrafiltration ceases. However, there is slow but continuous absorption of fluid via the peritoneal lymphatics, potentially leading to poor net ultrafiltration, low drain volume, and potential systemic volume expansion. Lower drain volumes could potentially lead to lower solute clearance.
If one were to design a peritoneal dialysis prescription based upon transport characteristics alone, ignoring patient convenience or lifestyle constraints, attempting to optimize ultrafiltration, drain volumes, and creatinine clearance, rapid transporters would do best with short dwell times (1.5 to 3 hours/dwell). In theory, when using only glucose-containing solutions, such patients would do best with automated therapies (automated peritoneal dialysis [APD]), such as nightly intermittent peritoneal dialysis (NIPD) or nightly cycler therapy with a last bag fill (first morning fill) and a midday exchange when only using glucose-containing solutions. By contrast, low-average or low transporters would do best with prolonged dwells, such as those associated with continuous ambulatory peritoneal dialysis (CAPD) or continuous cycling peritoneal dialysis (CCPD) with less overnight exchanges (figure 1). Low-average or low transporters may need large instilled volumes, which enhance clearance by maximizing contact surface area. In such patients, solute clearance and ultrafiltration tend to be maintained throughout the dwell because of the slow rates of transport for both small solutes and glucose. Patients who are high-average transporters would typically do well with either therapy.
Despite these theoretical concepts, most patients could do either CAPD or APD, if one is knowledgeable in peritoneal dialysis kinetics and peritoneal dialysis fluids and willing to individualize the patient's prescription. This is especially true in patients who have residual renal function, which makes it easier to maintain euvolemia and solute removal. In an extensive observational cohort review of 42,942 patients on CAPD and 23,439 on APD in the United States who started peritoneal dialysis during the years 1996 to 2004 and were followed through September 2006, there was no effect demonstrated of modality on survival risk . In this review, there was no adjustment for transport type. However, in a prior review of patients on peritoneal dialysis in Australia and New Zealand, which adjusted for transport type, there also was no demonstrable difference in risk of death for CAPD or APD . By contrast, an analysis of a more contemporary cohort of patients in peritoneal dialysis in Australia and New Zealand found a lower risk of death in high transporters treated with APD, compared with those on CAPD . At our center, approximately 85 percent of all peritoneal dialysis patients are on APD.
In addition, the removal of larger solutes such as B2-microglobulin is dwell-time dependant even among rapid transporters. Therefore, although these patients could often reach their minimal Kt/V urea targets with NIPD and a dry day, most are on 24 hours a day of peritoneal dialysis dwell. If they were not, they would not be optimizing middle-molecule removal due to the dry day.