Validation by computer simulation of two indirect methods for quantification of free water transport in peritoneal dialysis

Background: In peritoneal dialysis, approximately 40% of the total osmotic uttrafiftration (UF) induced by glucose can be predicted to be due to "free" water transport across aquaporin-1 (APQ-1). Theoretically, it would be possible to assess the fraction of free water transport in the early phase of a hypertonic dwell, when UF rate is high and the relative contribution of Na+ diffusion is tow. La Milia et al. [La Milia V. et al. Fast-fast peritoneal equilibration test (FAST-FAST-PET): a simple method for peritoneal hydraulic permeability study [Abstract]. Nephrol Dial Transplant 2002; 17 (Suppl 1):17-18] suggested a technique to assess sodium-associated water transport based on sodium removal (Na+R) divided by the plasma Na+ concentration during a "fast-fast" (60 minute) peritoneal equilibration test (PET) for 3.86% glucose, yielding an estimate of the UF passing through the small pores (UFSP). Free water transport (UF through ultrasmall pores; UFUSP) was obtained by subtracting UFSP from total UF Although peritoneal Na+ transport is almost totally convective, this technique will slightly overestimate small-pore UF due to the presence of some small-pore Na+ diffusion from the circulation during the dwelt. A way of dealing with this problem was presented recently by Smit (Smit W. et al. Quantification of free water transport in peritoneal dialysis. Kidney Int 2004; 66:849-854). Methods: In the present study we used the three-pore model of peritoneal transport to predict the degree of overestimation of UFSP for the technique presented by La Milia et al., and any potential deviations from theory for the technique presented by Smit et al. Simulations were performed under ordinary conditions and during simulated UF failure for 3.86% glucose. The fractional UF coefficient accounted for by APQ-1 was set at 2%. Results: Estimating the UFSP from the sodium-associated water transport according to the method by La Mitia et at. consistently overestimated UFSP and underestimated UFUSP. These errors were, however, minimal for dwells Lasting between 30 and 80 minutes. The technique by Smit et al. to calculate aquaporin-mediated water flow (UFUSP),