It is now well appreciated that peritoneal dialysis (PD) 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 (Figure 1). According to PET testing in different populations, about 15 percent of patients will be rapid transporters at the start of PD.
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 lead to lower solute clearance.
If one was to design a peritoneal dialysis prescription based upon transport characteristics alone, ignoring patient convenience or life style 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 (APD), such as nightly intermittent peritoneal dialysis (NIPD) or nightly cycler therapy with a last bag fill (first AM 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 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 PD kinetics, PD fluids and willing to individualize the patient's prescription. This is especially true in incident patients who have some residual renal function (making it easier to maintain euvolemia and solute removal). At our center, approximately 85 percent of all PD patients are on APD. In addition, the removal of larger solutes such as B2-microglobulin are 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 PD dwell. If they were not, they would not be optimizing middle molecule removal due to the dry day.
RAPID TRANSPORT AND MORTALITY
In addition to the potential for suboptimal ultrafiltration volumes and inadequate solute clearance, rapid transport status may be an independent predictor of enhanced mortality among patients undergoing continuous peritoneal dialysis [2-5]. This was shown in a 2006 meta-analysis of 20 observational studies, with 19 studies pooled to generate a summary mortality risk based upon transport status as defined by the ratio of creatinine in the dialysate to plasma after a standardized four-hour dwell (D/P) . For every 0.1 increase in the D/P value, there was an increase in the relative risk of death of 1.15 (95% CI 1.07-1.23). Thus, as compared to patients with low transport status, an increased mortality risk of 22, 46, and 77 percent was noted for low-average, high-average, and high transporters, respectively. In addition, there was a trend for an increased relative risk for death-censored technique failure with every 0.1 increase in the D/P value (1.18, 95% CI 0.96-1.46). (See "Adequacy of peritoneal dialysis".)