Long-term changes in solute and water transport.

The rate of transport of small solutes across the peritoneal membrane is one of the most important measurements in PD patients. Significant between-patient variability is associated with an impact on small solute clearance, ultrafiltration and even survival. Cross-sectional and longitudinal studies show that solute transport generally increases with time on treatment although this is again highly variable between individuals and is likely to represent an increased vascularity of the membrane. Initially, there is a coupled decrease in the ultrafiltration capacity of the membrane that can be explained by the earlier loss of the osmotic gradient leading predominantly to reduced free water transport via aquaporins combined with more fluid reabsorption once the osmotic gradient has dissipated. Subsequently, in some patients a further disproportionate fall in ultrafiltration occurs due to uncoupling of fluid transport from solute transport as a result of a reduction in the osmotic conductance of the membrane. Drivers of this damage appear to be peritonitis, glucose exposure and early loss of residual renal function. Cytokines and growth factors appear to be involved in this process and may prove useful biomarkers of membrane injury in the future.