Restriction coefficients of low molecular weight solutes and macromolecules during peritoneal dialysis

The intrinsic permeability of the peritoneal membrane can be functionally represented by the restriction coefficient (RC). The RC can be calculated as the exponent of the power relation between the mass transfer area coefficients (MTACs) of various solutes and their free diffusion coefficients in water. When the RC = 1.0, transport is determined by free diffusion only, as is expected for low molecular weight (LMW) solutes. A RC > 1.0 suggests that transport is restricted by the peritoneal membrane in a size-selective way, as has been found previously for macromolecules (MM). RCLMW can be calculated using the MTACs of urea, creatinine, urate, and beta 2-microglobulin, whereas RCMM can be calculated from clearances of beta 2-microglobulin, albumin, IgG, and alpha 2-macroglobulin. RCLMW and RCMM were determined in 108 peritoneal dialysis (PD) patients. In 36 patients, 3 or more (range 3-13) observations for RCLMW during a period of at least 2 years were available. RCMM were analyzed when present in the same patients. The median cross sectional values (n = 108) were: RCLMW: 1.22 (range 0.75-2.18) and RCMM: 2.30 (range 1.86-3.27). RCLMW was not correlated with time on PD, neither cross sectionally (r = -0.07, NS) nor after analysis of trend (mean regression coefficient t = 0.26, SD = 0.07). For RCMM a positive correlation with duration of PD was demonstrated (cross sectionally r = -0.18, p = 0.02, analysis of trend: t = 2.27, SD = 0.11, n = 27). Both RCs were not interrelated (r = -0.18, NS). The absence of a relation between both RCs suggests that LMW solutes and MM are transported by different pathways. The mean value of 1.22 for the RCLMW illustrates that the transport of LMW solutes is mainly by free diffusion, through the small-pore system. MM, which have to pass through the large-pore system, are restricted by the peritoneal membrane in a size-selective way, as shown by the high value of the RCMM. The lack of a correlation between the RCLMW and duration of PD indicates that no systematic changes occur in the small pores of the peritoneal vessels. In contrast, the increase of RCMM with duration of PD suggests restrictive changes at the level of the large-pore system.